Add Rails/DeprecatedActiveModelErrorsMethods rule

Even though we're already applying this rule since commit 08b12a78f,
it's very useful to have it so we don't accidentally introduce code that
won't work with Rails 7.

After upgrading to Rails 7, this rule will no longer be necessary, since
the code using the deprecated syntax will not work and so we'll notice
it immediately.

.rubocop.yml

@@ -377,6 +377,9 @@ Rails/CreateTableWithTimestamps:
 Rails/Date:
   Enabled: true
 
+Rails/DeprecatedActiveModelErrorsMethods: # TODO: Remove after upgrading to Rails 7
+  Enabled: true
+
 Rails/DurationArithmetic:
   Enabled: true