Make Ruby 2.7.6 the default for the GDK

What does this Merge Request do and why?

This MR makes Ruby 2.7.6 the default for the GDK by bringing in additional supporting scripts - support/asdf-exec and support/bundle-exec which both execute commands in the context of asdf to ensure the appropriate project's .tool-versions file is used.

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.

Steps to test

1. Ensure Ruby 2.7.6 is not installed
2. mkdir ~/tmp
3. cd ~/tmp && git clone git@gitlab.com:gitlab-org/gitlab-development-kit.git
4. cd gitlab-development-kit && git checkout 1580-fresh-gdk-one-line-install-fails
5. make bootstrap
6. make install
7. gdk start --open-when-ready

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 #1580 (closed)