Remove references to prior C# 8.0 and prior .netcore 3.1

Any guidelines that references to a specific approach for things prior to C# 8.0 and prior to .netcore 3.1 should be removed.

Search the CodingGuidelines repository for references to C# 8.0 and .NET Core 3.1, then read each affected guideline in context. Remove guidance that applies only to versions before those baselines, and verify that no prior-version references remain.

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