Add automation for merging dnup to release/dnup

I think this is a good idea since we need to ship out of release/dnup. This would allow us to work in the `dnup` branch while reducing effort needed to sync the release branch backup, and also allowing us to hotfix an existing release without including any other recent changes.

This is different than some of the other branch workflows since dnup only has one supported version at a time.

Author: nagilson

github-merge-flow.jsonc

@@ -35,6 +35,11 @@
         "release/10.0.2xx":{
             "MergeToBranch": "main",
             "ExtraSwitches": "-QuietComments"
+        },
+        // Automate opening PRs to merge sdk repos from dnup to release/dnup
+        "dnup":{
+            "MergeToBranch": "release/dnup",
+            "ExtraSwitches": "-QuietComments"
         }
     }
 }