Rename asdf-update to platform-update

What does this merge request do and why?

This MR renames the asdf-update Makefile target (and accompanying support script) to platform-update to handle all platform related logic (running brew, apt etc) as well as asdf logic if available and not opt'd out of.

How to set up and validate locally

1. Run make platform-update and ensure you see Updating asdf release, plugins, and tools (among other asdf related output)
2. Run gdk config set asdf.opt_out true
3. Re-run make platform-update and ensure you don't see any asdf related output

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.

Closes #1719 (closed)