CI: Cleanup Ruby 2 specific warnings used by `fail_on_warnings`

What does this MR do and why?

https://gitlab.com/gitlab-org/gitlab/-/blob/master/scripts/allowed_warnings.txt tracks allowed Ruby warnings.

Some of these warnings are specific to Ruby 2.7 and become unnecessary once we've dropped support for Ruby 2.

This MR drops all warnings related to Ruby 2.7

Closes #410480 (closed).

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