GitHub Actions doesn't have allow-failure. Use continue-on-error instead

Author: amatsuda

.github/workflows/ci.yml

@@ -47,13 +47,10 @@ jobs:
             gemfile: "gemfiles/Gemfile.rails-6.1.x.sprockets-4.x"
           - ruby: 2.7
             gemfile: Gemfile
-            allow-failure: true
           - ruby: '3.0'
             gemfile: Gemfile
-            allow-failure: true
           - ruby: '3.1'
             gemfile: Gemfile
-            allow-failure: true
           - ruby: head
             gemfile: Gemfile
 
@@ -69,3 +66,4 @@ jobs:
           bundler-cache: true
       - name: Run tests
         run: bundle exec rake
+        continue-on-error: ${{ matrix.gemfile == 'Gemfile' }}