Add Dependabot configuration for .NET SDK updates (#95)

This PR adds Dependabot configuration to automatically manage .NET SDK
version updates in global.json.

This configuration will:
- Monitor the global.json file for .NET SDK version updates
- Create pull requests when new SDK versions are available
- Help keep the project secure with the latest SDK patches

For more information, see:
https://devblogs.microsoft.com/dotnet/using-dependabot-to-manage-dotnet-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