* 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

* Improve RSS entries for statuses

- Render polls in both accounts and tags serializers
- Refactor RSS serializers
- Change title preview to include ellipsis when truncated
- Change title preview to show CW instead of toot text
- Add tests

* Remove title from OEmbed serialization

Twitter doesn't serialize title either, and tihs allows us to move the
title formatting code to the RSS serializers.