Move to .NET8

- Change TFM and LangVersion
- Better exception throwing (CA1510, CA1512, CA1513)
- Better exception utility method definition (CA1859)
- Prefer comparing `.Count` over calling `.Any()` (CA1860)
- Prefer `.AsSpan()` over `.Substring()` (CA1846)
- Add a few more `scoped`
- Use `RuntimeHelpers.GetUninitializedObject()` instead of `FormatterServices.GetUninitializedObject()`
- Use delegate instead of delegate pointer in variant generic conversions
- Enable EnforceExtendedAnalyzerRules in source generator projects
- Disable CS8981 on structs named movable in Godot.NativeInterop

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/Godot.SourceGenerators.Tests.csproj

@@ -1,9 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
-
-    <LangVersion>11</LangVersion>
+    <TargetFramework>net8.0</TargetFramework>
+    <LangVersion>12</LangVersion>
 
     <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>