Remove old rubies from CI

Ruby 2.x has been EOL'd for almost a year https://endoflife.date/ruby

It wasn't hard to keep track of compatibility with these older rubies,
but since they made CI fail in a way that's odd, and probably not
fixable, I am okay with not testing these in CI.

If we introduce an incompatibility somehow for an older Ruby, and you
run into it, let me know and I'm happy to cut another release that fixes
your issue.

.github/workflows/ci.yml

@@ -12,15 +12,7 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - 1.9.3
-          - '2.0'
-          - 2.1
-          - 2.2
-          - 2.4
-          - 2.5
-          - 2.6
-          - 2.7
-          - '3.0'
+          - 3.0
           - 3.1
           - 3.2
           - 3.3