Bring back Dependabot

Author: AArnott

.github/dependabot.yml

@@ -0,0 +1,21 @@
+# Please see the documentation for all configuration options:
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+- package-ecosystem: nuget
+  directory: /
+  schedule:
+    interval: weekly
+  groups:
+    nerdbank-gitversioning:
+      patterns:
+      - nbgv
+      - nerdbank.gitversioning
+    xunit:
+      patterns:
+      - 'xunit*'
+- package-ecosystem: dotnet-sdk
+  directory: /
+  schedule:
+    interval: monthly