ci: add v3 release branch to workflow triggers

Github needs to see all the triggers on the master/main branch version of this
file for them to work.

Signed-off-by: Christian Hopps <chopps@labn.net>

Author: choppsv1

.github/workflows/ci.yml

@@ -7,9 +7,11 @@ on:
   pull_request:
     branches:
       - master
+      - b_3.x
   push:
     branches:
       - master
+      - b_3.x
     tags:
       - v*