Add entity agnostic support

This pull request brings full compatibility with the latest diff module release. Noteworthy changes include:

1. Update the diff controller check to apply to more than just nodes
2. Fix previous / next URL generation for non-nodes
3. Fix cacheability issue that only manifests with block_content diffs (the diff_plus config must be a cacheable dependency)
4. Prevent WSOD if an entity type upcasts the left_revision or right_revision route parameters
5. Add a null check for the author (some of the newly supported entity types may not have authors set)
6. Update the template to account for content without canonical link-templates (block_content)
7. Update from annotations to attributes for various things (plugin definitions, test groups, etc...)
8. Add test coverage for a non-node (taxonomy, block content, and media should work)
9. Make Gitlab CI pass (2026 version).