Add .NET Framework 4.8 build target, remove .NET Core 2.1

[jasongin]

• Remove .NET Standard 2.0 and .NET Core 2.1 build and test targets, because .NET Core 2.1 is no longer supported.
• Add .NET Framework 4.8 build and test targets, conditional on Windows OS
• Change various code to be compatible with .NET Framework.
• Reduce test failures caused by unavailable crypto algorithms (ECDH, AES-GCM). The crypto tests specific to those algorithms will still fail, but other test cases will fallback to always-available algorithms. This reduces confusion about the cause for many test failures, particularly on Mac OS.
• Update YAML build definitions to target the appropriate .NET versions.

[jasongin]

.NET Core 3.1 went out of support just last month, and .NET 5 only has four months left. But I don't see a need to remove those just yet.