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

app/views/admin/dashboard/index.html.haml

@@ -5,7 +5,7 @@
   .flash-message-stack
     - @system_checks.each do |message|
       .flash-message.warning
-        = t("admin.system_checks.#{message.key}.message_html", message.value ? { value: content_tag(:strong, message.value) } : {})
+        = t("admin.system_checks.#{message.key}.message_html", value: message.value ? content_tag(:strong, message.value) : nil)
         - if message.action
           = link_to t("admin.system_checks.#{message.key}.action"), message.action