* 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