Add dotnet-sdk to Dependabot configuration

Author: BenjaminMichaelis

.github/dependabot.yml

@@ -9,3 +9,10 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "dotnet-sdk"
+    # Look for a `global.json` in the `root` directory
+    directory: "/"
+    # Check for updates once a week
+    schedule:
+      interval: "weekly"