chore(standards): bump TargetRubyVersion to 3.4

The dev-toolchain container now ships Ruby 3.4.9 (v1.9.0 release).
Update the recommended `.rubocop.yml` example so consumers default to
the matching TargetRubyVersion.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: matthew-on-git

content/docs/standards/ruby.md

@@ -37,7 +37,7 @@ require:
   - rubocop-performance
 
 AllCops:
-  TargetRubyVersion: 3.1
+  TargetRubyVersion: 3.4
   NewCops: enable
   Exclude:
     - "db/schema.rb"