Update Gitpod image to use mise

What does this merge request do and why?

Update Gitpod image to use mise. asdf support in GDK is being removed asap.

Fixes #2773 (closed)

How to set up and validate locally

1. Create a new gitpod from this merge request: gitlab!198126 (merged)
2. Be patient
3. Profit!

I took a video of my workspace:

gitpod workspace using mise

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 MR references an issue describing the change.
• 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.