Commit to ruby@2.6 support

Closes [hotwired#681][]

First, remove `ruby@3.0` syntax including `...` arguments and end-less
method definitions.

Next, add `ruby@2.6` and `ruby@2.7` to the CI matrix along with
`rails@7.1`.

[hotwired#681]: hotwired#681

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
 
       - name: Run Bug Template Tests
         run: ruby bug_report_template.rb || ruby bug_report_template.rb
-        continue-on-error: ${{ startsWith(matrix.ruby, '2') || false }}
+        continue-on-error: ${{ startsWith(matrix.ruby, "2") || false }}
 
       - name: Run tests
         id: test