[Infra] Add .ruby-version file for GHA

[ncooke3]

Context

• Pin the monorepo's Ruby version to 2.7 (for use by GHA)
• From https://github.com/ruby/setup-ruby#usage:

- uses: ruby/setup-ruby@v1
      with:
        ruby-version: '3.0' # Not needed with a .ruby-version file

#no-changelog

[ncooke3]

@paulb777, after some experiments in #9919, I think the .ruby-version file needs to be checked into master to be picked up by GHA. It should be possible based on actions/setup-ruby#31 (comment)

Can I try merging this PR so I can then confirm that setting the ruby-version in each YML file isn't needed anymore?

[ncooke3]

Cool, this did the trick. Verified here: https://github.com/firebase/firebase-ios-sdk/runs/6943204414?check_suite_focus=true#step:4:14

[ryanwilson]

As a heads up: this broke my local CocoaPods flow since I had version 2.7.1 installed and not 2.7.0:

$ pod repo update
rbenv: version `ruby-2.7' is not installed (set by <REDACTED_PATH_TO_REPO>/.ruby-version)

I'm not sure the best solution, but wanted to call it out in case others run into this.

[ncooke3]

Sorry @ryanwilson, I had a feeling something like that would happen! Were you able to easily pin your version to 2.7.0?

[ryanwilson]

Sorry @ryanwilson, I had a feeling something like that would happen! Were you able to easily pin your version to 2.7.0?

It was easier for me to just change that value to my local version (2.7.1) 😝