branch-rules.yml: kgraphviewer kf6-stable is 2.5 now, no more kf5 branches

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.