Remove update-from-default-branch job

What does this merge request do and why?

This MR disables the verify:update-from-default-branch CI job because it's prone to failure where there's a bug in the main branch. For example, !3057 (merged) was an MR to fix a bug in main but we couldn't easily merge the fix MR because of the update-from-default-branch job blocking progress.

The verify:update-from-* jobs perform the following:

• verify:update-from-default-branch - Clone GDK repo and checkout the GDK main branch, perform a gdk install, checkout the MR branch and finally perform a gdk update.
• verify:update-from-mr-branch - Clone GDK repo and checkout the MR branch, perform a gdk install and finally perform a gdk update.

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.