Add Ruby 3.0 support (#16046)

* Fix issues with POSIX::Spawn, Terrapin and Ruby 3.0

Also improve the Terrapin monkey-patch for the stderr/stdout issue.

* Fix keyword argument handling throughout the codebase

* Monkey-patch Paperclip to fix keyword arguments handling in validators

* Change validation_extensions to please CodeClimate

* Bump microformats from 4.2.1 to 4.3.1

* Allow Ruby 3.0

* Add Ruby 3.0 test target to CircleCI

* Add test for admin dashboard warnings

* Fix admin dashboard warnings on Ruby 3.0

config/initializers/session_store.rb

@@ -1,7 +1,6 @@
 # Be sure to restart your server when you modify this file.
 
-Rails.application.config.session_store :cookie_store, {
+Rails.application.config.session_store :cookie_store,
   key: '_mastodon_session',
   secure: (Rails.env.production? || ENV['LOCAL_HTTPS'] == 'true'),
-  same_site: :lax,
-}
+  same_site: :lax