Rip out support for end-of-life Ruby versions

We've been carrying support for a bunch of EOL Ruby versions on the
basis that it was easy. Adding support for Ruby 4.0 means adding `cgi`
as an explicit dependency as it is no longer a default gem. The latest
version of it is incompatible with JRuby 9.1, which targets Ruby 2.x
compatibility (while JRuby targets more specific Ruby versions now,
before JRuby 9.2, it followed whatever the latest version of 2.x was).

We might be able to find a way to conditionally avoid installing the gem
on that version of JRuby and let it use the included version, but at
this point I don't want to pile on more hacks to support EOL versions of
Ruby that have been out of support for a long time. There are better
places to spend effort on this library.

Let's take the opportunity to officially drop all currently EOL Ruby
versions. I'll bump the major version in the next release.

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

Author: Sinjo

.circleci/config.yml

@@ -12,26 +12,7 @@ jobs:
 
     steps:
     - checkout
-    # We need to limit the version of Rubygems that we update to for some of the older
-    # JRuby versions. Specifically:
-    #
-    #   - JRuby 9.2.x targets Ruby 2.5, and Rubygems 3.3.26 was the last version to
-    #     support that
-    #   - JRuby 9.3.x targets Ruby 2.6, and version 3.4.22 was the last version to
-    #     support that
-    #
-    # For now, we'll pin to the last supported version. We can drop this check once we
-    # drop all those versions of JRuby.
-    - run: |
-        running_very_old_ruby=$(ruby -e 'puts Gem::Version.new(RUBY_VERSION) < Gem::Version.new("2.6.0")')
-        running_moderately_old_ruby=$(ruby -e 'puts Gem::Version.new(RUBY_VERSION) < Gem::Version.new("3.0.0")')
-        if [[ "${running_very_old_ruby}" == "true" ]]; then
-          gem update --system 3.3.26
-        elif [[ "${running_moderately_old_ruby}" == "true" ]]; then
-          gem update --system 3.4.22
-        else
-          gem update --system
-        fi
+    - run: gem update --system
     - run: bundle install
     - run: bundle exec rake
 
@@ -43,21 +24,7 @@ workflows:
         matrix:
           parameters:
             ruby_image:
-            - cimg/ruby:2.6
-            - cimg/ruby:2.7
-            - cimg/ruby:3.0
-            - cimg/ruby:3.1
             - cimg/ruby:3.2
             - cimg/ruby:3.3
             - cimg/ruby:3.4
-            # We've switched to JRuby's own Docker image as CircleCI decided to
-            # stop supporting it when they deprecated their legacy images. For
-            # some reason, the official JRuby image for 9.1 fails with an
-            # inscrutable error message that isn't worth debugging.
-            #
-            # We'll drop this version on our next major release anyway, as it's
-            # out of security support.
-            - circleci/jruby:9.1
-            - jruby:9.2
-            - jruby:9.3
             - jruby:9.4