Add Style/HashExcept Rubocop rule

We were already using it because Rails adds the `Hash#except` method for
Ruby 2.7 and earlier, but since the method wasn't part of Ruby itself,
the Rubocop rule only works with Ruby 3.0 and later.

.rubocop.yml

@@ -505,6 +505,9 @@ Style/FileWrite:
 Style/HashConversion:
   Enabled: true
 
+Style/HashExcept:
+  Enabled: true
+
 Style/HashSyntax:
   Enabled: true