chore: drop dependency on ruby2_keywords

[Earlopain]

Now that Ruby below 2.7 is not supported (#389, 38b083a) this is not needed anymore

[linux-foundation-easycla]

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[xuan-cao-swi]

Since you are trying to remove ruby2_keywords dependencies, maybe also need to remove them in active_record/patches/persistence_insert_class_methods.rb

[Earlopain]

The ruby2_keywords methods are natively defined in ruby versions after 2.6, all this dependency does is define empty shims for compatibility with older versions (see https://github.com/ruby/ruby2_keywords/blob/3882dac4c9bc784183d7d4cf7374edd56017746f/lib/ruby2_keywords.rb)

Removing these from existing code would change behaviour in some way and I can't say what that entails. It would be better kept for a separate PR.