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.

.packit.yaml

@@ -4,7 +4,7 @@
 specfile_path: rpm/dtkcore.spec
 
 # add or remove files that should be synced
-synced_files:
+files_to_sync:
     - rpm/dtkcore.spec
     - .packit.yaml