* Remove obsolete RSS::Serializer test
Since #17828, RSS::Serializer no longer has specific code for deleted statuses,
but it is never called on deleted statuses anyway.
* Rename erroneously-named test files
* Fix failing test
* Fix test deprecation warnings
* Update CircleCI Ruby orb
1.4.0 has a bug that does not match all the test files due to incorrect
globbing
