Use DateTime.now instead of Date.today when computing age of last gdk commit for out-of-date check

What does this merge request do and why?

Updates the "your GDK is out-of-date" check to use DateTime.now instead of Date.today when doing the math to check how many days old a commit is.

Mixing Date and DateTime seemed to be making the math weird. I don't think this bug has a significant functional impact, beyond that for some reason the test was failing on my local, even in main. This code is only used for knowing when to make a comparison to the latest main commit, and for displaying the message associated with the out-of-date warning.

The spec also makes more sense with this change. The spec sets up a 10 day old commit, which now reports 10 days old (Previously 9. Or sometimes 8).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.