Since #30016 Rails generates `.ruby-version` file
in order to help Ruby version manager tools like `rbenv`, `rvm`
determine which Ruby version should be used for the current Rails
project.

Since #32649 Rails sets Ruby version to the file compatible with MRI/JRuby
by default.

Pull Request #31496 reports that `.ruby-version` doesn't match ruby version other
than stable version and recommends to use `ENV["RBENV_VERSION"]`, and
`ENV["rvm_ruby_string"]` in order to set correct Ruby version to the file
that `rbenv` or `rvm` can understand.
Also, there is another similar issue that reports the same case if use
JRuby https://github.com/jruby/jruby/issues/5144.

Closes #31496, https://github.com/jruby/jruby/issues/5144.