Drop .NET 5 and .NET 7 support

[Corniel]

as .NET 5 is out of support almost 2 years, and .NET 7 will be in May, it is time to drop support, as it will reduce the number of target dependent statements. This will be part of the Qowaiv 7.0 release.

See: https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core