Update Style/HashSyntax Rubocop rule

Ruby 3.1 adds the option for hash shortcuts, so it's possible to write
`{ user: , poll: }` instead of `{ user: user, poll: poll }`.

By default, Rubocop expects the new syntax in Ruby 3.1. While right now
I absolutely hate this new syntax, we're allowing both the old and the
new styles because we might start adopting it once we get used to it.

.rubocop.yml

@@ -646,6 +646,7 @@ Style/HashExcept:
 
 Style/HashSyntax:
   Enabled: true
+  EnforcedShorthandSyntax: either
 
 Style/HashTransformKeys:
   Enabled: true