Unify the server name variable between the charts

[keyvaann]

Some of the charts use serverName and some other use server_name. We should use only one of these variables to avoid confusion.

➜  radar-helm-charts git:(downgrade-radar-appserver) grep -r serverName                                                                                                            git:(downgrade-radar-appserver|) 
charts/radar-rest-sources-authorizer/README.md:74:| serverName | string | `"localhost"` | Domain name of the server |
charts/radar-rest-sources-authorizer/values.yaml:160:serverName: localhost
charts/radar-rest-sources-authorizer/templates/deployment.yaml:52:            value: {{ printf "%s://%s/rest-sources/backend/" $https .Values.serverName }}
charts/radar-rest-sources-authorizer/templates/deployment.yaml:62:            value: {{ printf "%s://%s/rest-sources/authorizer/login" $https .Values.serverName }}
charts/radar-rest-sources-authorizer/templates/deployment.yaml:64:            value: {{ printf "%s://%s/managementportal/oauth" $https .Values.serverName }}
charts/radar-upload-connect-backend/README.md:79:| serverName | string | `"localhost"` | Server name or domain name |
charts/radar-upload-connect-backend/values.yaml:194:serverName: localhost
charts/radar-upload-connect-backend/templates/configmap.yaml:11:    advertisedBaseUri: {{ printf "%s://%s/upload/api" $https .Values.serverName }}
charts/kube-prometheus-stack/values-defaults.yaml:1168:      serverName: kubernetes
charts/kube-prometheus-stack/values-defaults.yaml:1510:      serverName: null
charts/kube-prometheus-stack/values-defaults.yaml:1726:    ##   serverName: localhost
charts/kube-prometheus-stack/values-defaults.yaml:1762:      serverName: ""
charts/kube-prometheus-stack/values-defaults.yaml:1872:      serverName: null
charts/kube-prometheus-stack/values-defaults.yaml:4089:      # serverName: ""
charts/radar-rest-sources-backend/README.md:84:| serverName | string | `"localhost"` | Resolvable server name, needed to find the advertised URL and callback URL |
charts/radar-rest-sources-backend/values.yaml:235:serverName: localhost
charts/radar-appserver/README.md:83:| serverName | string | `"localhost"` |  |
charts/radar-appserver/values.yaml:203:serverName: localhost
charts/radar-appserver/templates/configmap.yaml:20:        - {{ printf "%s://%s/managementportal/oauth/token_key" $https .Values.serverName | quote }}
charts/app-config-frontend/README.md:74:| serverName | string | `"localhost"` | Resolvable server name, needed to find the advertised URL and callback URL |
charts/app-config-frontend/values.yaml:159:serverName: localhost
charts/app-config-frontend/templates/deployment.yaml:56:              value: {{ printf "%s://%s/managementportal/oauth" $https .Values.serverName }}
charts/app-config-frontend/templates/deployment.yaml:58:              value: {{ printf "%s://%s/appconfig/login" $https .Values.serverName }}


➜  radar-helm-charts git:(downgrade-radar-appserver) grep -r server_name                                                                                                           git:(downgrade-radar-appserver|) 
charts/radar-upload-connect-frontend/README.md:73:| server_name | string | `"localhost"` | Server name or domain name |
charts/radar-upload-connect-frontend/values.yaml:159:server_name: localhost
charts/radar-upload-connect-frontend/templates/deployment.yaml:52:            value: {{ printf "%s://%s/upload/api" .Values.server_name}}
charts/radar-upload-connect-frontend/templates/deployment.yaml:54:            value: {{ printf "%s://%s/managementportal/oauth" .Values.server_name }}
charts/radar-upload-connect-frontend/templates/deployment.yaml:56:            value: "{{ .Values.server_name }}/upload/login"
charts/kube-prometheus-stack/values.yaml:1:server_name: localhost
charts/radar-rest-sources-backend/templates/configmap.yaml:14:      advertisedBaseUri: {{ printf "%s://%s/rest-sources/backend/" $https .Values.server_name }}
charts/management-portal/README.md:83:| server_name | string | `"localhost"` | domain name of the server |
charts/management-portal/README.md:86:| identity_server.server_url | string | `nil` | The publicly accessible server URL for the IDP; needed when deviating from http(s)://server_name/kratos |
charts/management-portal/README.md:88:| identity_server.login_url | string | `nil` | The publicly accessible login URL for the IDP; needed when deviating from http(s)://server_name/kratos-ui |
charts/management-portal/values.yaml:266:server_name: localhost
charts/management-portal/values.yaml:274:  # -- The publicly accessible server URL for the IDP; needed when deviating from http(s)://server_name/kratos
charts/management-portal/values.yaml:278:  # -- The publicly accessible login URL for the IDP; needed when deviating from http(s)://server_name/kratos-ui
charts/management-portal/templates/deployment.yaml:74:            value: {{ printf "%s://%s" $https .Values.server_name }}
charts/management-portal/templates/deployment.yaml:76:            value: {{ printf "%s://%s/managementportal" $https .Values.server_name }}
charts/management-portal/templates/deployment.yaml:93:            value: {{ default (printf "%s://%s/kratos" $https .Values.server_name) .Values.identity_server.server_url }}
charts/management-portal/templates/deployment.yaml:95:            value: {{ default (printf "%s://%s/kratos-ui" $https .Values.server_name) .Values.identity_server.login_url }}
charts/management-portal/templates/secrets-config.yaml:16:    https://{{ $.Values.server_name }}{{ $redirect_uri }}