Un-pin json gem and update to latest

NOTE: there is a change in the underlying behavior of
`JSON.pretty_generate` such that an empty array will now generate `[]`
where before there were two newlines (`\n`) between the brackets like
```
[

]
```

The result of this is that the `RenderedJobInstance#configuration_hash`
for jobs which have empty links _will change_ even though there are no
changes to these jobs.

src/Gemfile

@@ -9,7 +9,6 @@ gem 'bosh_common', path: 'bosh_common'
 gem 'mysql2'
 gem 'pg'
 gem 'sequel', '~> 5.29.0'
-gem 'json', '~> 2.7.0' # TODO to unpin fix bosh-director: rspec ./spec/unit/job_renderer_spec.rb:84
 
 group :development, :test do
   gem 'bundle-audit'

src/Gemfile.lock

@@ -177,7 +177,7 @@ GEM
     io-endpoint (0.14.0)
     io-event (1.7.3)
     io-stream (0.6.1)
-    json (2.7.6)
+    json (2.8.0)
     language_server-protocol (3.17.0.3)
     little-plugger (1.1.4)
     logger (1.6.1)
@@ -355,7 +355,6 @@ DEPENDENCIES
   bundle-audit
   factory_bot
   fakefs
-  json (~> 2.7.0)
   minitar
   mysql2
   parallel_tests

src/bosh-director/spec/unit/job_renderer_spec.rb

@@ -79,7 +79,7 @@ module Bosh::Director
           JobRenderer.render_job_instances_with_cache(per_spec_logger, [instance_plan], cache, encoder, link_provider_intents)
 
           expect(instance_plan.rendered_templates.template_hashes.keys).to eq ['dummy']
-          expect(instance.configuration_hash).to eq('53b2a7cac279af8bb73885b08a794ab5dd21bb8c')
+          expect(instance.configuration_hash).to eq('d4712f01af58824081e9d964522782da9889cd1b')
           expect(instance.template_hashes.keys).to eq(['dummy'])
         end