Use GitHub Issue Labeler action instead of dotnet-issue-labeler

[MichaelSimons]

We don't get very much value from the automatic labels provided by https://github.com/dotnet/issue-labeler. I can make an argument it is adding overhead because it keeps labeling issues with area-labels that have been deleted (therefore recreating them).

This repo should move to a simpler approach using the GitHub issue labeler action to apply the untriaged label to all issues, and leave area-labels up to our issue triage process.