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.