org.kde.Sdk: fix branch versions to match branch id

The current branch is 5.9 yet the values specified in finish-args point
at a non-existant master branch. This fixes those to also point at version
5.9 like the Platform/Sdk.

Without this, applications such as GNOME Builder will try to download the
org.kde.Sdk//master runtime instead of 5.9 as they resolve the SDK version
from the runtime metadata.