Improve CI

- Test against multiple Ruby version
- Experimental tests against ruby-head to spot issues in advance

.github/workflows/ruby.yml

@@ -13,13 +13,23 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby-version: ['3.1', '3.2', '3.3']
+        channel: ['stable']
+
+        include:
+          - ruby-version: 'head'
+            channel: 'experimental'
+
+    continue-on-error: ${{ matrix.channel != 'stable' }}
 
     steps:
     - uses: actions/checkout@v4
-    - name: Set up Ruby 2.7
+    - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7.7
+        ruby-version: '3.3'
         bundler-cache: true
     - name: Test with Rake
       run: |