chore: add .promu.yml for restore upstream for alertmanager

Signed-off-by: Jayapriya Pai <janantha@redhat.com>

Author: slashpai

.github/workflows/merge-alertmanager.yaml

@@ -23,6 +23,7 @@ jobs:
       restore-upstream: >-
          CHANGELOG.md
          VERSION
+         .promu.yml
          go.mod
          go.sum
       assets-cmd: |