Require parser 3.0
Require parser 3.0

This updates the dependency on parser to ~> 3.0.0.

Allowing parser 3.0 is necessary to allow concurrent installation with
RuboCop 1.8.0.

Versions below 3.0 are disallowed because:

- On Ruby 3.0, users should not install Parser versions below 3.0, since
  they do not officially support 3.0's syntax.
- Parser 3.0 supports all versions of Ruby that Reek supports.

troessner/reek,troessner/reek