Revert test matrix to Ruby 2.7 only

The lockfiles are generated for Ruby 2.7. Testing with multiple Ruby
versions requires regenerating all lockfiles with the proper Ruby
version, which is out of scope for this PR.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

.github/workflows/ruby.yml

@@ -69,7 +69,7 @@ jobs:
             installer: pnpm
           - name: bun
             installer: bun
-        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3']
+        ruby: ['2.7']
         gemfile:
           # These have shakapacker:
           - base