Don't test against MySQL on Ruby 2.3

GitHub Actions has issues, and it isn't worth solving:
ruby/setup-ruby#150

.github/workflows/ci.yml

@@ -22,6 +22,11 @@ jobs:
           - rails70
         db: [POSTGRES, MYSQL, SQLITE]
         exclude:
+          # MySQL has issues on Ruby 2.3
+          # https://github.com/ruby/setup-ruby/issues/150
+          - ruby: 2.3
+            db: MYSQL
+
           # Rails 5.0 supports Ruby 2.2-2.4
           - appraisal: rails50
             ruby: 2.5