Drop Ruby 2.6 support

[kupolak]

Description

Drop support for Ruby 2.6. Adresses #1693

Type of change

• Reducing technical debt

Please add an entry to the relevant section of CHANGELOG.md as part of this pull request. - Done ✔️

Note to other contributors

This PR might fix the red build here: #1689

Update required of cucumber.io/docs

Changes are not required in cucumber.io/docs

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code - not needed
• New and existing tests are passing locally and on CI - tested locally, i was not able to trigger the CI. But it should work, we are only removing the language version from matrix
• bundle exec rubocop reports no offenses
• RDoc comments have been updated - not needed
• CHANGELOG.md has been updated

[olleolleolle]

@kupolak Minor: Will we need to re-run RuboCop after re-configuring it to target 2.7?

[olleolleolle]

One thing:

• JRuby 9.3.x is compatible with Ruby 2.6.x - and if we are removing 2.6, it ought to be replaced with jruby-9.4

[luke-hill]

I'm happy for the change to jruby in the target matrix as specified by Olle.

@kupolak if you make that change and get CI green I'll approve and merge this.

[olleolleolle]

Looks good from here!

[aslakhellesoy]

Hi @kupolak,

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