[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
The command line arguments to `configure` are stored in `rbconfig.rb`.
Usually these are just useless but harmless after the installation,
however could affect unexpectedly when updating default gems.
