Only enable auto-merge in main (dotnet#56932)

* Only enable auto-merge in main

* Update resourceManagement.yml

Author: wtgodbe

.github/policies/resourceManagement.yml

@@ -234,6 +234,8 @@ configuration:
       - titleContains:
           pattern: Update dependencies
           isRegex: False
+      - targetsBranch:
+          branch: main
       then:
       - addLabel:
           label: area-infrastructure
@@ -252,6 +254,8 @@ configuration:
       - titleContains:
           pattern: Merge branch
           isRegex: False
+      - targetsBranch:
+          branch: main
       then:
       - addLabel:
           label: area-infrastructure