Upgrade from .sln to .slnx solution files

[Joy-less]

In April 2024, Microsoft added support for a new solution file format called .slnx to replace the old .sln format.

.slnx uses XML similar to .csproj.

The difference in readability is huge. Okay, it doesn't really matter that much but I think it's cool.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Joy-less]

I see MSBuild doesn't support this yet, this can be delayed until .slnx gets better support.

[linkdotnet]

In some other repositories of mine, I also support the slnx file. Because of tooling support and what not (and it is still marked as experimental) I would prefer if we keep both versions.
I don't expect many changes to the SLN files anyway, so the maintenance cost isn't that high (I hope).

[linkdotnet]

The GitHub Pipeline is failing. I guess they don't support slnx as of now

[linkdotnet]

Given that this is not experimental anymore since SDK version 9.0.200, we can proceed with this one. I will rebase onto the current mainline branch

EDIT: Can't. @Joy-less could you rebase and therefore trigger another run?

[Joy-less]

EDIT: Can't. @Joy-less could you rebase and therefore trigger another run?

I'll do this later today. ^-^

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud