[action] Only add 'automerge' label on master version upgrade PR. (sonic-net#15112)

Why I did it
Don't auto update package version for release branch.

Work item tracking
Microsoft ADO (number only): 22335854
How I did it
How to verify it

Author: liushilongbuaa

.github/workflows/label.yml

@@ -13,9 +13,6 @@ on:
     - reopened
     branches:
     - master
-    - '202012'
-    - '202[1-9][0-9][0-9]'
-    - '20[3-9][0-9][0-9][0-9]'
     paths:
     - 'files/build/versions/**'