Add rubocop-rake support

What does this Merge Request do and why?

This MR adds support for rubocop-rake as suggested when running rubocop:

$ make rubocop
--snip--
245 files inspected, no offenses detected

Tip: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-rake (https://rubygems.org/gems/rubocop-rake)

You can opt out of this message by adding the following to your config (see https://docs.rubocop.org/rubocop/extensions.html#extension-suggestions for more options):
  AllCops:
    SuggestExtensions: false

We also fix new offences picked up by the inclusion of rubocop-rake.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.