[Infra] Update dependabot config to enable automated dependency submission (open-telemetry#6813)

Author: martincostello

.github/dependabot.yml

@@ -1,8 +1,10 @@
 # This file is retained solely for automated tooling to see we do automated
 # dependency updates as not all such scanners recognize the use of Renovate.
+# It also works around a limitation that Automatic dependency submission does
+# not automatically detect .slnx files to signify the use of .NET in a repo.
 version: 2
 updates:
-  - package-ecosystem: "github-actions"
+  - package-ecosystem: "nuget"
     directory: "/"
     schedule:
       interval: yearly