Modernize GitHub Actions workflow

Update to latest action versions and enable bundler caching
for faster CI builds. Move Ruby version specification to
.ruby-version file for consistency across development
and CI environments.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: mostlyobvious

.github/workflows/ruby.yml

@@ -12,10 +12,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v2
-    - uses: actions/setup-ruby@v1
+    - uses: actions/checkout@v4
+    - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.6.x
+        bundler-cache: true
     - name: Test
       run: |
         ./bin/setup

.ruby-version

@@ -0,0 +1 @@
+4.0.1