Parallelize gdk update

What does this merge request do and why?

This MR speeds up gdk update by using rake’s multitask for update tasks that can run in parallel.

Related to #2115 (closed)

Demo

Before	After
2024-08-26_13-45-08	2024-08-26_13-43-32

Time difference

One-off sample, without self-update, 2nd time running gdk update:

Before	After
2:44.75	1:55.44

How to set up and validate locally

1. Run GDK_SELF_UPDATE=0 gdk update
2. See that the update is parallel and faster

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.