Simplify projects

[0xced]

• Update minimum required .NET SDK to version 5.0.302
• Change netcoreapp2.2 (out of support) to netcoreapp2.1 (LTS)
• Remove AssemblyInfo.cs and replace with equivalent csproj properties
• Remove properties with default values (such as AssemblyName and PackageId)
• Remove duplication in csproj files by testing on $(TargetFrameworkIdentifier) instead of $(TargetFramework)
• Don't define SYSTEM_NET and MAIL_KIT and use the build-in NETSTANDARD define instead when required
• Update test packages to their latest versions
• Use NETCOREAPP3_1_OR_GREATER instead of defining a custom NEW_ARGUMENTOUTOFRANGEEXCEPTION_MESSAGE
• Add .NET 5.0 target framework in test project

[nblumhardt]

Thanks for this @0xced - unfortunately I haven't been able to find much time for this project, so since you sent this we've been through three more major .NET versions! I've had a shot at much the same thing (and made breaking changes liberally) in #122. Thanks again though and sorry for this silence on this one :-)