Set Mend branch to master

Previously, the Mend action referenced 7.x and main branches. These
branches do not exist in this repository and were likely copied over
from the puppet or puppet-agent repositories.

This commit updates Mend to reference the master branch, which is the
default branch for puppet-runtime.

.github/workflows/mend.yaml

@@ -3,8 +3,7 @@ name: Mend Monitor
 on:
   push:
     branches:
-      - 7.x
-      - main
+      - master
 jobs:
   mend_monitor:
     if: ${{ github.repository_owner == 'puppetlabs' }}
@@ -33,4 +32,3 @@ jobs:
           WS_USERKEY: ${{ secrets.MEND_TOKEN }}
           WS_PRODUCTNAME: Puppet Agent
           WS_PROJECTNAME: ${{ github.event.repository.name }}
-