Make GitMaintenance diagnostic more failure resilient

What does this merge request do and why?

This MR makes GitMaintenance more resilient by raising an error with the stderr contents if the git config Shellout fails.

Closes #2157

How to set up and validate locally

1. To simulate a failing command, change git config in lib/gdk/diagnostic/git_maintenance.rb to git confi
2. Run gdk doctor
3. See an error like:

   Git Maintenance recommendation
   ================================================================================
   git config failed: git: 'confi' is not a git command. See 'git --help'.
   
   The most similar command is
           config

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.