Make MRs build using Ruby 3.1 by default

Status Quo

Our pipeline schedules currently are set up as follows:

https://gitlab.com/gitlab-org/gitlab/-/pipeline_schedules

• MRs are built using Ruby 3.0
• It is possible to add a label to build an MR using Ruby 3.1

Proposal

As we move towards rolling out Ruby 3.1, we'd like to start:

• Building every MR using Ruby 3.1
• Ensure if engineers wish to run the MR with Ruby 3.0 then the label pipeline:run-in-ruby3_0 works as expected for this

CI builds would continue to use Ruby 3.0 for now until we move to the roll out of Ruby 3.1

Reasoning

This will give more focus to ensuring that MR are build/tested with Ruby 3.1, whilst the 2 hourly and CI builds running at 3.0 mean that changes are still being built and tested on the platform we currently deploy to (Ruby 3.0)