Add Dependabot configuration for .NET SDK updates (#677)

Author: JamieMagee

.github/dependabot.yml

@@ -10,7 +10,6 @@ updates:
     #   - assignee_one
     # reviewers:
     #   - reviewer_one
-
   - package-ecosystem: "nuget"
     # location of package manifests
     directory: "/"
@@ -24,5 +23,14 @@ updates:
     #   - assignee_one
     # reviewers:
     #   - reviewer_one
-
+  - package-ecosystem: dotnet-sdk
+    directory: /
+    schedule:
+      interval: weekly
+      day: wednesday
+    ignore:
+      - dependency-name: '*'
+        update-types:
+          - version-update:semver-major
+          - version-update:semver-minor
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