Drop Ruby 2.6 runtime support

[ydah]

This PR drops Ruby 2.6 runtime support. The next release will be the time to drop Ruby 2.6 support:

https://www.ruby-lang.org/en/downloads/branches/ https://docs.rubocop.org/rubocop/compatibility.html#support-matrix

Follow up: https://github.com/rubocop/rubocop/pull/11791

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with main (if not - rebase it).
• [x] Squashed related commits together.
• [-] Added tests.
• [-] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

[ydah]

@bquorning Could you removed the Ruby 2.6 part of our main branch protection?

[bquorning]

Yep, I adjusted the branch protection.

[ydah]

Thank you❤️