Change default port for registry to 5100 to avoid macOS conflict

What does this merge request do and why?

This MR changes the default port for the registry from 5000 to 5100 to avoid conflicts with macOS Control Center, so macOS users don’t have to manually update the port in GDK.

Closes #2479 (closed)

How to set up and validate locally

1. GDK_SELF_UPDATE=0 gdk update
2. See the announcement

3. Enable registry with gdk config set registry.enabled true
4. gdk reconfigure
5. gdk restart registry
6. curl -i registry.test:5100
7. See a 200 OK response

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.