added required_ruby_version >= 2.7 into gemspec

[skunkworker]

With the recent constraint of dropping support for Ruby 2.6 (which also includes JRuby 9.3), the minimum ruby version should be constrained in the gemspec so that it cannot be resolved for older versions.

In addition I would recommend re-releasing 4.0.0, 4.0.1, 4.0.2 gems so that 3.1.0 would be pulled in correctly.

[wspurgin]

Thanks @skunkworker I definitely should've added it in the gemspec in the first place, and I'll cop to that.

However, I can't republish versions to rubygems. It's disallowed (for natural security reasons). The most I could do is yank previous versions which I don't particularly want to do either. Since it's in the change log and release notes that we dropped support for <2.7 (and attempting to use an older Ruby will produce breakages), I'll let the previous versions stand.

Thanks for the PR!