Using tags instead of branch names is currently not supported.

Just, the last kf5 releases of kgraphviewer have been from master branch,
which is now KF6-only.
The only other project depending on kgraphviewer is massif-visualizer,
whose last stable kf5 branch does not even have ci setup.

So just removing the entries for @latest & @stable (doable also as there
is no graphics/* glob), which in case someone tries to acquire those
old variants the error seen should be as early as possible and be clear.