GHA: Publish snapshots for 3.9.x

Change-Id: I3d1c3df280b7f588266753b1d7517f3818bb899e
Reviewed-on: https://review.couchbase.org/c/couchbase-jvm-clients/+/232869
Tested-by: David Nault <[email protected]>
Reviewed-by: David Nault <[email protected]>
(cherry picked from commit 002cdca)
Reviewed-on: https://review.couchbase.org/c/couchbase-jvm-clients/+/232876

Author: dnault

.github/workflows/deploy-snapshot.yml

@@ -7,6 +7,7 @@ on:
   push:
     branches:
       - master
+      - 3.9.x
     paths-ignore:
       - '.gitignore'
       - '.editorconfig'