[GDK] Make Ruby 3 the default Ruby for the GDK

What does this Merge Request do and why?

This MR aims to make Ruby 3 the default version for the GDK only.

Related: !2686 (merged)

How to set up and validate locally

Ruby 3.0.5 should already be installed as it's in https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/.tool-versions already.

⚠ Stop any existing GDKs you might have running, you can check by running ps -ef | grep runsvdir and inspecting the <GDK-dir>/services part.

⚠ Also ensure your ~/tmp/gitlab-development-kit directory doesn't exist.

mkdir ~/tmp
[[ -d ~/tmp/gitlab-development-kit ]] && (echo "ERROR: ~/tmp/gitlab-development-kit already exists, please remove before proceeding" ; exit 1)
cd ~/tmp && git clone git@gitlab.com:gitlab-org/gitlab-development-kit.git
cd gitlab-development-kit && git checkout 1158-gdk-make-ruby-3-the-default-ruby
make bootstrap
make install
gdk start --open-when-ready

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1158 (closed)