Add Dependabot configuration for .NET SDK updates

Author: JamieMagee

.github/dependabot.yml

@@ -11,4 +11,14 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
-      day: "sunday"
+      day: "sunday"
+  - package-ecosystem: dotnet-sdk
+    directory: /
+    schedule:
+      interval: weekly
+      day: wednesday
+    ignore:
+      - dependency-name: '*'
+        update-types:
+          - version-update:semver-major
+          - version-update:semver-minor