Centralize Ruby Version to .ruby-version

[jenshenny]

What are you trying to accomplish?

The .ruby-version file is the ecosystem standard for defining a Ruby version. This PR adds the .ruby-version file, and removes all other references to Ruby in this repository, aligning it with the standard.

What should reviewers focus on?

Important

Please verify the following before merging:

Verify that the changes in the PR meets the following requirements or adjust manually to make it compliant:

• .ruby-version file is present with the correct Ruby version defined
• There is no Ruby version/requirement referenced in the Gemfile (no lines with ruby <some-version>)
• A Gemfile.lock is built with the defined Ruby version
• There is no TargetRubyVersion defined in rubocop.yml
• There is no Ruby argument present in ruby/setup-ruby Github Actions that do not run on a Ruby matrix (no lines with ruby-version: “x.x”)

Please merge this PR if it looks good, this PR will be merged if there isn't any activity after 4 weeks.

[k0kubun]

For yjit-bench, we need to constantly use multiple Ruby versions. Can we avoid locking the Ruby version in this repository? I don't want .ruby-version to inadvertently switch Ruby from master to 3.3.0 when I enter yjit-bench directory, for example.

[jenshenny]

For yjit-bench, we need to constantly use multiple Ruby versions. Can we avoid locking the Ruby version in this repository? I don't want .ruby-version to inadvertently switch Ruby from master to 3.3.0 when I enter yjit-bench directory, for example.

Ah gotcha, thanks for the context - I think it makes sense not to add a .ruby-version in this case then. For that reason, I'll close this.