Requires Ruby 2.7+

[sue445]

I want to add Prism gem as runtime dependency, but Prism gem requires Ruby 2.7+

https://rubygems.org/gems/prism/versions/1.4.0

Because every version of rspec-parameterized-core depends on prism >= 0
  and prism >= 0.22.0 depends on Ruby >= 2.7.0,
every version of rspec-parameterized-core requires Ruby >= 2.7.0 or prism >=
0, < 0.22.0.
And because prism < 0.22.0 depends on Ruby >= 3.0.0,
  every version of rspec-parameterized-core requires Ruby >= 2.7.0.
So, because Gemfile depends on rspec-parameterized-core >= 0
  and current Ruby version is = 2.6.10,
  version solving has failed.

https://github.com/rspec-parameterized/rspec-parameterized-core/actions/runs/14791209614/job/41528572607

So I dropped Ruby < 2.7

rer. #17, #18