Add Dependabot configuration for .NET SDK updates (#66)

Author: JamieMagee

.github/dependabot.yml

@@ -16,7 +16,6 @@ updates:
       - dependencies
     reviewers:
       - azure/psrule-rules-azure
-
   # Maintain dependencies for Dev Containers
   - package-ecosystem: devcontainers
     directory: '/'
@@ -26,3 +25,13 @@ updates:
       - dependencies
     reviewers:
       - azure/psrule-rules-azure
+  - package-ecosystem: dotnet-sdk
+    directory: /
+    schedule:
+      interval: weekly
+      day: wednesday
+    ignore:
+      - dependency-name: '*'
+        update-types:
+          - version-update:semver-major
+          - version-update:semver-minor