Delete notes via API

Supports deleting issues, snippets, and merge requests via the API.

• Closes #14944 (closed)
• Closes #14845 (closed)
• Closes #6060 (closed)

@zj I did not see that you assigned yourself in #6060 (closed). Hopefully, you did not start yet.

@rymai In #6060 (closed) this is targeted for 8.7 release. Could you review that and maybe this still lands in 8.7.