ci: Run pipelines for the `ruby3` branch

What does this MR do and why?

In the ruby3 branch, we modified the CI config to run pipelines for the ruby3 branch, and defined two variables there.

I think we can actually add this condition to the master branch config, and sets the variable under this condition only.

After this is merged, we should be able to remove the hardcoded

  RUBY_VERSION: "3.0"
  NOTIFY_PIPELINE_FAILURE_CHANNEL: "f_ruby3"

change from the ruby3 branch.

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