Update ruby version in validation workflow

Author: jwheare

.github/workflows/validation.yml

@@ -12,7 +12,7 @@ jobs:
     - name: Set up Ruby
       uses: actions/setup-ruby@v1
       with:
-        ruby-version: 2.6
+        ruby-version: 2.7.7
 
     - name: Build
       run: |