Promote experimental quiet mode to supported

What does this Merge Request do?

Thi MR promotes the gdk.experimental.quiet setting to be supported by making it available at gdk.quiet and enabled by default:

gdk:
  quiet: true

!1267 (merged) is there the setting was originally added where you can see which commands the setting affects.

Related: !1267 (merged)

Merge Request checklist1

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.