Define policy for updates to Roslyn version

[mthalman]

There should be a written policy for updating the Roslyn version in the aspnet Dockerfiles (see #786 (comment)). This should define when updates will occur, what versions will be used, as well as a general description of the intent for making any version changes. This policy should be incorporated as concrete tasks in the release template to ensure it's captured as part of the regular release workflow.

[Dotnet-GitSync-Bot]

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