.NET SDK 3 is end of life (eol) - move to 6 or 7

[peterhoeg]

The last SDK bump was back in Oct 2022: #1852

The SDK is eol as per https://dotnet.microsoft.com/en-us/download/dotnet/3.1#sdk-3.1.424

This means it's no longer possible to build on distributions that aggressively removed unsupported software.

[bergmeister]

There is already a PR for it: #1873

[peterhoeg]

I had searched before opening this, but missed it.