chore: drop Ruby 3.1, add 3.4 to CI matrix

Ruby 3.1 reached EOL March 2025. Update CI matrix to 3.2, 3.3, 3.4
and bump RuboCop TargetRubyVersion to 3.2.

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

Author: markhallen

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby-version: ['3.1', '3.2', '3.3']
+        ruby-version: ['3.2', '3.3', '3.4']
 
     steps:
     - uses: actions/checkout@v4

.rubocop.yml

@@ -8,7 +8,7 @@ plugins:
   - rubocop-rake
 
 AllCops:
-  TargetRubyVersion: 3.1
+  TargetRubyVersion: 3.2
   NewCops: enable
   Exclude:
     - "vendor/**/*"