Remove view fragment caching for project READMEs

What does this MR do?

It only cached the rendered README and no longer both the plain README and view fragment.

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

It also should work in case the README was removed: #29594 (closed)

Why was this MR needed?

It could happen the README view fragment was not updated due to caching.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26585 (closed), #29594 (closed)