* 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
* No need to re-require sidekiq plugins, they are required via Gemfile
* Add derailed_benchmarks tool, no need to require TTY gems in Gemfile
* Replace ruby-oembed with FetchOEmbedService
Reduce startup by 45382 allocated objects
* Remove preloaded JSON-LD in favour of caching HTTP responses
Reduce boot RAM by about 6 MiB
* Fix tests
* Fix test suite by stubbing out JSON-LD contexts
Claire