API: Moved `DELETE /projects/:id/star` to `POST /projects/:id/unstar`

What does this MR do?

Moves the unstar API endpoint from DELETE to POST to make it more restful.

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 https://gitlab.com/gitlab-org/gitlab-ce/issues/28328

changed milestone to %9.0

added ~13884 breaking change labels

added 291 commits

• c2e0dcf4...12cd4c83 - 290 commits from branch master
• 378aab30 - API: Moved DELETE /projects/:id/star to POST /projects/:id/unstar

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assigned to @rymai

added 16 commits

• 378aab30...c89449e6 - 15 commits from branch master
• a9ccd3bd - API: Moved DELETE /projects/:id/star to POST /projects/:id/unstar

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assigned to @razer6

resolved all discussions

assigned to @rymai

added 1 commit

• 039c6d60 - API: Moved DELETE /projects/:id/star to POST /projects/:id/unstar

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@rymai I updated the MR.

added ~164274 ~889916 labels

@razer6 Awesome work, thank you!

merged

mentioned in merge request !11449 (merged)