Adding Microsoft SECURITY.MD

[microsoft-github-policy-service[bot]]

Please accept this contribution adding the standard Microsoft SECURITY.MD :lock: file to help the community understand the security policy and how to safely report security issues. GitHub uses the presence of this file to light-up security reminders and a link to the file. This pull request commits the latest official SECURITY.MD file from https://github.com/microsoft/repo-templates/blob/main/shared/SECURITY.md.

Microsoft teams can learn more about this effort and share feedback within the open source guidance available internally.

[mgoertz-msft]

This has opened up a can of worms - after updating the github actions it now complains that .NET 4.5.1 developer pack is not available, which is not that surprising considering that .NET 4.5.1 has been out of support since 2016. Looks like the oldest supported version now is 4.6.2: https://docs.microsoft.com/en-us/lifecycle/faq/dotnet-framework