Bring back dependabot

Author: AArnott

.github/dependabot.yml

@@ -19,3 +19,7 @@ updates:
   directory: /
   schedule:
     interval: monthly
+  ignore:
+  - dependency-name: Microsoft.CodeAnalysis* # We intentionally target older VS versions.
+  - dependency-name: Microsoft.Bcl.AsyncInterfaces # We want to match the minimum target .NET runtime
+  - dependency-name: System.Collections.Immutable # We want to match the minimum target .NET runtime