update rubocop configuration for CI

Author: rei-moo

.rubocop.yml

@@ -2,3 +2,22 @@ inherit_gem:
   rubocop-github:
     - config/default.yml
 require: rubocop-performance
+
+AllCops:
+  TargetRubyVersion: 2.6
+
+# Disable cops that are not consistently available across all Ruby versions
+Style/ClassMethodsDefinitions:
+  Enabled: false
+
+Style/OrAssignment:
+  Enabled: false
+
+GitHub/AvoidObjectSendWithDynamicMethod:
+  Enabled: false
+
+Layout/SpaceInsideHashLiteralBraces:
+  Enabled: false
+
+Lint/ParenthesesAsGroupedExpression:
+  Enabled: false