Lower RuboCop TargetRubyVersion to 2.4 to match test_gemfiles matrix

The per-Rails gemfile matrix in .github/workflows/test_gemfiles.yml
runs specs on Ruby versions down to 2.4 (for the AR 5.0-5.2 jobs),
but .rubocop.yml had TargetRubyVersion set to 3.2. With that higher
target, RuboCop would have permitted Ruby 3.2+ syntax / stdlib usage
that would break the older runtimes actually covered by CI.

Lower TargetRubyVersion to 2.4 so lint matches the minimum supported
Ruby.

Addresses review feedback from
#239 (comment)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yahonda

.rubocop.yml

@@ -1,5 +1,9 @@
 AllCops:
-  TargetRubyVersion: 3.2
+  # Match the minimum Ruby exercised by the per-Rails matrix in
+  # .github/workflows/test_gemfiles.yml (AR 5.0-5.2 jobs run on Ruby 2.4)
+  # so RuboCop does not permit syntax/stdlib usage that would break
+  # those supported runtimes.
+  TargetRubyVersion: 2.4
   DisabledByDefault: true
   NewCops: disable