Use serialize_keys as part of highlight diff cache key

What does this MR do?

It makes highlight diff cache key dependent on keys used in the cached value - this assures that old/invalid cache is not used if we change the format of cached data.

Are there points in the code the reviewer needs to double check?

It was not possible to add new diff comments (see https://gitlab.com/gitlab-org/gitlab-ce/issues/48801)

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #48801 (closed)