Remove deprecated templates endpoints in V4

What does this MR do?

Remove the following deprecated endpoints on V4 (and also remain these untouched on V3):

- /licences
- /licences/:key
- /gitignores
- /gitlab_ci_ymls 
- /dockerfiles
- /gitignores/:key
- /gitlab_ci_ymls/:key
- /dockerfiles/:key

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

/dockerfiles and /dockerfiles/:name also have been deprecated on V3 and removed on V4.

---

• 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?

EE-backport: gitlab-ee!1214

Closes #22818 (closed)