chore(deps): require Ruby >=2.4.0

[DirtyF]

Align with other core plugins that already require 2.4
Ruby 2.4 is now EOL.

[ashmaroli]

From what I recall, we have not started enforcing Ruby 2.4 in core plugin's gemspec if the library code doesn't use any Ruby 2.4+ specific syntax..

[DirtyF]

It's more about following Ruby release cycle, and require a maintained Ruby release.

You're right the current codebase style is OK even we were to target Ruby 2.7, but we didn't bother looking if we could use the latest Ruby additions.

[DirtyF]

https://www.ruby-lang.org/en/news/2020/04/05/support-of-ruby-2-4-has-ended/

[ashmaroli]

I'm still of the opinion that this change is not necessary — there's no explicit need to use this plugin with Ruby 2.4 and above. But there's already an approval, so perhaps I'm not seeing the value..

[DirtyF]

@jekyll: merge +minor