Drop Ruby 2.6 support

[koic]

Follow up rubocop/rubocop#11791.

This PR drops Ruby 2.6 support. And JRuby has been dropped from the CI matrix because no JRuby-specific issues have been found and prevents the following build error:

#!/bin/bash -eo pipefail
bundle install
Fetching https://github.com/rubocop/rubocop.git
You need to install git to be able to use gems from git repositories. For help
installing git, please refer to GitHub's tutorial at
https://help.github.com/articles/set-up-git

Exited with code exit status 11
CircleCI received exit code 11

https://app.circleci.com/pipelines/github/rubocop/rubocop-minitest/926/workflows/68776b54-5856-4acd-a99a-f41df114634d/jobs/6034

This does not mean no support for JRuby. RuboCop Rails will continue to support JRuby 9.4+.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.