chore: remove obsolete Stride.ProjectGenerator tool and update_solutions.bat

The tool generated platform-specific .sln files from Stride.sln using
<StrideBuildTags> filtering and .Windows.csproj renaming. Both workflows
are obsolete:
- Platform solutions are now .slnf filters on Stride.slnx
- Cross-platform targets are handled by multi-TFM in a single .csproj

The project-unittest subcommand generated old-style multi-platform test
project scaffolding (.Windows.csproj, AndroidManifest.xml, etc.) which
is incompatible with the current SDK-style build system.

Also removes:
- build/update_solutions.bat (sole caller of the tool)
- Stride.ProjectGenerator entry from build/Stride.slnx
- /sources/tools/Stride.ProjectGenerator/build/ from .gitignore
- Stale generator reference from Stride.Graphics.Tests/README.md

Author: Kryptos-FR

.gitignore

@@ -53,7 +53,6 @@ Cache/
 /externals/
 /sources/common/externals/
 /sources/tools/Stride.ShaderKeyGenerator/build/
-/sources/tools/Stride.ProjectGenerator/build/
 /sources/tools/SLConverter/BuildRelease/Package/SLConverter-*.pdf
 /sources/tools/SLConverter/BuildRelease/Package/bin/x64/SLConverter.exe
 /sources/tools/SLConverter/BuildRelease/Package/bin/x86/SLConverter.exe

build/Stride.slnx

@@ -292,10 +292,7 @@
     <Project Path="../sources/tools/Stride.Graphics.RenderDocPlugin/Stride.Graphics.RenderDocPlugin.csproj">
       <Build Solution="*|Win32" Project="false" />
     </Project>
-    <Project Path="../sources/tools/Stride.ProjectGenerator/Stride.ProjectGenerator.csproj">
-      <Build Solution="*|Win32" Project="false" />
-    </Project>
-    <Project Path="../sources/tools/Stride.StorageTool/Stride.StorageTool.csproj">
+<Project Path="../sources/tools/Stride.StorageTool/Stride.StorageTool.csproj">
       <Build Solution="*|Win32" Project="false" />
     </Project>
   </Folder>

build/update_solutions.bat

@@ -3,5 +3,3 @@
 To add/remove/update tests for all platforms, check the associated Stride.Graphics.Tests.Shared project.
 To add/remove/update tests for a specific platform, add them here and make sure to add the Label "Stride.DoNotSync" as in:
     <Compile Label="Stride.DoNotSync" Include="MyPlatformSpecificType.cs" />
-
-Do not modify the project files as they are automatically generated by the ProjectGenerator tool, update the tool instead.