Build/Test tools: Unify REST API group as `restapi`.

Replace `@group rest-api` with `@group restapi` to unify the tests under the one group name, chosen on the basis that majority rules.

Follow up to r61045, r61032.

See #64225.



git-svn-id: https://develop.svn.wordpress.org/trunk@61999 602fd350-edb4-49c9-b593-d223f7449a82