Bump the required BASERUBY version to 2.5 (#7504)

[Misc #16671]

I'd like to bump it to 2.7 to use pattern matching in
tool/mk_builtin_loader.rb.

However, I experienced a few blockers. 2.5 seems like the closest
version that is easy enough to use on CIs, so let me bump the version to
it as an intermediate step for it. I want to use &. and <<~ in 2.3 too.

Known blockers:
* AppVeyor Visual Studio 2015 doesn't have Ruby 2.7. You'd need to bump
  the version to Visual Studio 2019.
* GitHub Actions windows-2019 doesn't have Ruby 2.7 either. You
  can use ruby/setup-ruby, but configure doesn't seem to work with it.
* For ruby/ruby-ci-imaage, bionic doesn't have Ruby 2.7. I tried using
  ruby-build to build Ruby 2.7 from package, but the build on its CI
  seems to somehow loop forever when I do that. So I gave it up for now.
  We might want to wait until bionic becomes EOL.

Note:
* AppVeyor Visual Studio 2015 has Ruby <= 2.6.3
  https://www.appveyor.com/docs/windows-images-software/#ruby
* GitHub Actions windows-2019 uses Ruby 2.5.9
  https://github.com/actions/runner-images/blob/main/images/win/Windows2019-Readme.md

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