Test all files with Rubocop, not just added code in the diff

[mkllnk]

What? Why?

• Closes [Rubocop] Change linter to run Rubocop on the whole codebase #12389

Generally, we want to keep the whole code base healthy. If a PR changes the Rubocop rules, we want the whole codebase to be tested. For example, when we upgrade Rubocop, we now don't have to run it locally any more, CI will do it for us. We also found that the diff filter didn't always work as expected.

What should we test?

• No test.

Release notes

Changelog Category (reviewers may add a label for the release notes):

• User facing changes
• API changes (V0, V1, DFC or Webhook)
• Technical changes only
• Feature toggled

The title of the pull request will be included in the release notes.

Dependencies

Documentation updates

[dacook]

And with Bundler too, good one!

I had at the bottom of my to-do list to try "option 3", which could be more efficient. But I'm glad you saved me from going down the rabbit-hole, besides as you noted, diffing isn't perfect.