Fix "master"->"3.x" for 2.19/CI too (being default branch gets used)

Author: cowtowncoder

.github/workflows/upstream-dispatch-v3.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Checkout current branch (full)
         uses: actions/checkout@v4
         with:
-          ref: master
+          ref: 3.x
           fetch-depth: 0
 
       - name: Setup sbt