Use fabricbot to automatically apply untriaged label when issues are created

[jeffhandley]

The dotnet/machinelearning repository needs to have untriaged automatically applied when new issues are created. This will then tie into the rest of the automation we use fabricbot for in that repository. We should also apply that automation here in the fabricbot-config repository and discuss with the docs team if we should adopt untriaged in the dotnet-api-docs repository too.

For the dotnet/runtime repository, we use issue-labeler to automatically apply the untriaged label. But at least for repositories that don't use issue-labeler, we should use fabricbot to automatically apply the untriaged label to new issues. We should consider decommissioning that feature of issue-labeler and exclusively using fabricbot to serve that purpose.

• [x] Create a task in the fabricbot-config scripts to allow a repository to opt into having untriaged applied automatically when new issues are created
• [x] Opt the fabricbot-config into using that task
• [x] Opt the machinelearning repo into using that task
• [x] Discuss with the docs team if we should opt the dotnet-api-docs repo into using that task
• [x] Determine if we should retire that behavior out of issue-labeler and replace it with the fabricbot task for it; if so, migrate all repos that rely on issue-labeler for the task to use fabricbot