ci: Prevent Upload Reports job from running on release branches

This won't work, since the workflows on the backbranches are structured differently
and thus the job can't find the loadtest.md file anywhere.

.github/workflows/report.yaml

@@ -5,6 +5,8 @@ on:
     workflows:
       - CI
       - Test
+    branches-ignore:
+      - v[0-9]+
     types:
       - completed