Change main to be .NET Monitor 9 for dependabot updates (#8199)

Author: jander-msft

.github/dependabot.template.yml

@@ -17,7 +17,7 @@
 #! target branch and which major version of .NET Monitor it builds.
 #@ def getBranches():
 #@   return [
-#@     struct.encode({"name": "main", "majorVersion": "8"}),
+#@     struct.encode({"name": "main", "majorVersion": "9"}),
 #@     struct.encode({"name": "release/9.0", "majorVersion": "9"}),
 #@     struct.encode({"name": "release/8.x", "majorVersion": "8"}),
 #@   ]

.github/dependabot.yml

@@ -34,6 +34,23 @@ updates:
   target-branch: main
   commit-message:
     prefix: '[main] '
+- package-ecosystem: nuget
+  directory: /eng/dependabot/net9.0
+  schedule:
+    interval: daily
+  target-branch: main
+  ignore:
+  - dependency-name: '*'
+    update-types:
+    - version-update:semver-major
+  commit-message:
+    prefix: '[main] '
+  groups:
+    runtime-dependencies:
+      patterns:
+      - Microsoft.Extensions.*
+      - Microsoft.NETCore.DotNetHost
+      - System.Text.Json
 - package-ecosystem: nuget
   directory: /eng/dependabot/net8.0
   schedule: