[Try to] use SLNX format for solution files

[jamescrosswell]

See https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/

• Would need to tweak some of our scripts.
• Need to see if there is any equivalent of SLNF files as well (filtered solution files).

[aritchie]

This is not recommended yet. There is still some tooling issues and it offers nothing from a functionality standpoint at this time

[jamescrosswell]

I'll put a v6.0 milestone on this. We can review when we look at the next major release (likely required to support .NET 10 late this year or early next year).

[jamescrosswell]

Looks like this will be the default in .NET 10 and yes it does support solution filters.