Fixes the incoherent filtering by milestone in `/projects/:id/issues` and `/groups/:id/issues` endpoints

Originally opened at !7134 (closed) by @eisnerd.

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Changing /projects/:id/issues and /issues to use the IssuesFinder changes the meaning of the labels parameter, so the tests have been changed to match. This could be a slight gotcha for existing API users. The resources gain some parameters that are implemented in the IssuesFinder but not documented in the API documentation or covered by the API tests. These could become part of the API simply by adding docs and tests, but this has not been done in this MR.

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

Closes #23194 (closed)