Update Go in .tool-versions to 1.22.6

What does this merge request do and why?

Go 1.22.5 doesn't work with XCode 16 due to a code signing issue (https://github.com/golang/go/issues/68088) that has been fixed in Go 1.22.6. Bump the GDK version to ensure binaries are built with Go 1.22.6.

Relates to #2223 (closed)

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.