Unify issues search behavior by always filtering when ALL labels matches
As explained on #26315 (closed), this MR unifies the way we filter issues by labels by always returning issues that matches ALL given labels. This also leads to simpler and reusable code/behavior.
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Changelog entry added -
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API support added - Tests
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All builds are passing
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Conform by the merge request performance guides -
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Branch has no merge conflicts with master
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Squashed related commits together
Closes #26315 (closed) EE-backport gitlab-ee!1258