ci: enable jobs for the 3.x stable branch

In order to trigger the deploy job on push on the stable branch, change
the trigger conditions.

Signed-off-by: Robin Jarry <robin@jarry.cc>

Author: rjarry

.github/workflows/ci.yml

@@ -6,10 +6,10 @@ name: CI
 on:
   pull_request:
     branches:
-      - master
+      - b_3.x
   push:
     branches:
-      - master
+      - b_3.x
     tags:
       - v*