Add dotnet-sdk to dependabot.yml.

Author: alexrp

.github/dependabot.yml

@@ -1,6 +1,15 @@
 # yaml-language-server: $schema=https://json.schemastore.org/dependabot-2.0.json
 version: 2
 updates:
+  - package-ecosystem: dotnet-sdk
+    directories:
+      - /
+    schedule:
+      interval: daily
+      timezone: Europe/Copenhagen
+      time: "12:00"
+    open-pull-requests-limit: 25
+    labels: []
   - package-ecosystem: github-actions
     directories:
       - /