Fix test-related issues (#17888)

* 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

spec/lib/rss/serializer_spec.rb

@@ -13,13 +13,6 @@ describe RSS::Serializer do
 
     subject { RSS::Serializer.new.send(:status_title, status) }
 
-    context 'if destroyed?' do
-      it 'returns "#{account.acct} deleted status"' do
-        status.destroy!
-        expect(subject).to eq "#{account.acct} deleted status"
-      end
-    end
-
     context 'on a toot with long text' do
       let(:text) { "This toot's text is longer than the allowed number of characters" }