Drop support for ruby 2.7

[nobu]

Also 2.7 is EOL for 2.5 years, I think such warnings or minor inconveniences are OK, and it would only affect 2.7 users keeping pp updated. So maybe it would be fine to do nothing?

I encountered test failures by this warning at bigdecimal with ruby 2.7.

Well, I agree that 2.7 support will not matter nowadays, and propose the dropping it, like as 2-4 in https://github.com/ruby/pp/issues/51#issuecomment-3368162006.

So new plan is:

1. release 0.6.3, the final version for ruby 2.
2. bump up to 0.7.0.
3. drop the support for ruby 2.
4. revert https://github.com/ruby/pp/pull/53 (optional).

Originally posted by @nobu in #51

[nobu]

I'm not sure if push_gem.yml works for tags on commits that aren't the HEAD. Should #58 be split into the first commit and the others?