Remove support for Ruby 2.6 (#21477)

As pointed out by https://github.com/mastodon/mastodon/pull/21297#discussion_r1028372193
at least one of our dependencies already dropped support for Ruby 2.6, and we
had removed Ruby 2.6 tests from the CI over a year ago (#16861).

So stop advertising Ruby 2.6 support, bump targeted version, and drop some
compatibility code.

.rubocop.yml

@@ -2,7 +2,7 @@ require:
   - rubocop-rails
 
 AllCops:
-  TargetRubyVersion: 2.5
+  TargetRubyVersion: 2.7
   NewCops: disable
   Exclude:
     - 'spec/**/*'