Migrate SLN to SLNX

[Code-Grump]

🤔 What's changed?

Converted our primary solution file to the new SLNX format.

⚡️ What's your motivation?

This keeps us up-to-date with current .NET technology and gives us all the benefits of the new SLNX format.

🏷️ What kind of change is this?

• :bank: Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

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[304NotModified]

Please also update the docs E.g.

https://github.com/search?q=repo%3Areqnroll%2FReqnroll%20sln&type=code

The minimum VS is now 2019?

[Code-Grump]

Ah yeah. The contrib guide has to be updated.

Done