Move make update to rake update

What does this merge request do and why?

This moves the update make target to a new rake task called update in lib/tasks/update.rake.

This MR is intended to not change the behavior of gdk update (except for errors, where the message changes). That will happen in follow up MRs (e.g. in Add support for tty-spinner in rake (!3814 - merged) and for Parallelize gdk update (#2115 - closed)).

How to set up and validate locally

From make

1. Run make update
2. See the deprecation warning
3. See that GDK updates as before

From gdk

1. Run GDK_SELF_UPDATE=0 gdk update
2. See that GDK updates as before

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.