Fix configuration for Packit 1.0.0

This commit fixes the configuration for the forthcoming Packit 1.0.0.
See [our blog post](https://packit.dev/posts/packit_1_0_0_action_required) for more details.
  - Job type `build` has been changed to `copr_build`.
  - Job type `production_build` has been changed to `upstream_koji_build`.
  - Key `upstream_project_name` has been changed to `upstream_package_name`.
  - Key `synced_files` has been changed to `files_to_sync`.

Please review and merge me before January 2025 otherwise packit-service jobs will fail because of an invalid configuration.

rhel-only: ci

Resolves: RHEL-70884

.packit.yml

@@ -4,7 +4,7 @@
 # Docs: https://packit.dev/docs/
 
 specfile_path: .packit_rpm/systemd.spec
-synced_files:
+files_to_sync:
   - .packit.yaml
   - src: .packit_rpm/systemd.spec
     dest: systemd.spec