API: Version information

What does this MR do?

Adds a new endpoint to retrieve the version information.

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

Since I don't use an entity but only provide a hash, I cannot specify the return entity in the grape documentation. Is there a different way to do that?

I added this to 8.14 since I think it won't get merged into 8.13. If yes, I'm happy to change that. Edit: Added to 8.13.

Why was this MR needed?

Clients can now use this information to enable/disable certain API features depending on the version.

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 #22608 (closed), https://gitlab.com/gitlab-org/gitlab-ce/issues/23148