Update README to add MacOS 14 to supported versions.

What does this merge request do and why?

While trying to setup GDK, the README mentions MacOS is supported only until version 13. But in fact GDK is supported in MacOS 14. I've added Version 14 to supported platforms in README.

How to set up and validate locally

Can pull the branch and check the README.

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.