.NET: Downgrade Microsoft.CodeAnalysis.CSharp to 4.8.0 for compat with SDK 8.0.1xy

[akien-mga]

And for CI, set up .NET SDK 8.0.100 explicitly to test our min supported version.

Should fail now due to #100502, and we can use it to validate the upcoming fix.

• Fixes .NET: Building GodotSharp is broken in current master branch #100502.

[akien-mga]

Finally got the CI config to work. I couldn't make global.json work properly, so instead I'm just removing the preinstalled .NET SDKs before installing 8.0.100.

The CI run for 21ca022 properly fails, confirming #100502: https://github.com/godotengine/godot/actions/runs/12375025460/job/34538922837?pr=100506

CSC : warning CS9057: The analyzer assembly '/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/Godot.SourceGenerators.Internal/bin/Debug/netstandard2.0/Godot.SourceGenerators.Internal.dll' references version '4.9.0.0' of the compiler, which is newer than the currently running version '4.8.0.0'. [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]
Error: /home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/Core/NativeInterop/NativeFuncs.cs(7,13): error CS0234: The type or namespace name 'SourceGenerators' does not exist in the namespace 'Godot' (are you missing an assembly reference?) [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]
Error: /home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/Core/NativeInterop/NativeFuncs.cs(18,6): error CS0246: The type or namespace name 'GenerateUnmanagedCallbacksAttribute' could not be found (are you missing a using directive or an assembly reference?) [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]
Error: /home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/Core/NativeInterop/NativeFuncs.cs(18,6): error CS0246: The type or namespace name 'GenerateUnmanagedCallbacks' could not be found (are you missing a using directive or an assembly reference?) [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]
Error: /home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/Core/NativeInterop/NativeFuncs.cs(185,36): error CS8795: Partial method 'NativeFuncs.godotsharp_variant_new_copy(out godot_variant, scoped in godot_variant)' must have an implementation part because it has accessibility modifiers. [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]
Error: /home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/Core/NativeInterop/NativeFuncs.cs(319,36): error CS8795: Partial method 'NativeFuncs.godotsharp_string_name_new_copy(out godot_string_name, scoped in godot_string_name)' must have an implementation part because it has accessibility modifiers. [/home/runner/work/godot/godot/modules/mono/glue/GodotSharp/GodotSharp/GodotSharp.csproj]

...

[akien-mga]

And https://github.com/godotengine/godot/actions/runs/12375157412/job/34539363154 confirms that 5232760 solves the issue.

I'll squash the two commits now so this is ready to merge.

[paulloz]

Thanks, sorry for missing this in the original PR.