Stop running license scan pipeline on preview branches after release

[ellahathaway]

The license scan pipeline ran for the 10.0.1xx-preview6 branch today: https://dev.azure.com/dnceng/internal/_build/results?buildId=2754461&view=results. This is because the pipeline is scheduled to run Mondays at 7am UTC. However, preview 6 was released last week, so the pipeline should no longer run on this branch. Similarly, because there were failures in the pipeline, a PR targeting the preview6 branch was created. This PR is not valid because we already released preview6.

[MichaelSimons]

[Triage] The only way to prevent this is to delete the branch. If that is desirable, it should be done as part of the release pipeline.

[ellahathaway]

These branches can be deleted after their respective releases. I’ve filed https://github.com/dotnet/release/issues/1400 to track this work. Closing this issue in favor of that.