Upgrade to .NET 9

[PJB3005]

Right now the update to .NET 9 is effectively blocked on the following Rider bugs:

• https://youtrack.jetbrains.com/issue/RIDER-120193/Code-analysis-cant-infer-type-arguments-involving-implicit-casts-code-compiles-fine
• https://youtrack.jetbrains.com/issue/RIDER-120208/Code-analysis-error-for-LINQ-Aggregate-method-on-.NET-9

We would either need to work around these by changing our code, or just hope Jetbrains fixes them soon:tm:.

Otherwise, the launcher needs to be updated first:

• Update to .NET 9 SS14.Launcher#193

And of course server hosts should be notified when we flick the switch.