nuget: bump the silk-net group with 3 updates

Bumps the silk-net group with 3 updates: [Silk.NET.Vulkan](https://github.com/dotnet/Silk.NET), [Silk.NET.Vulkan.Extensions.EXT](https://github.com/dotnet/Silk.NET) and [Silk.NET.Vulkan.Extensions.KHR](https://github.com/dotnet/Silk.NET).


Updates `Silk.NET.Vulkan` from 2.16.0 to 2.21.0
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](dotnet/Silk.NET@v2.16.0...v2.21.0)

Updates `Silk.NET.Vulkan.Extensions.EXT` from 2.16.0 to 2.21.0
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](dotnet/Silk.NET@v2.16.0...v2.21.0)

Updates `Silk.NET.Vulkan` from 2.16.0 to 2.21.0
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](dotnet/Silk.NET@v2.16.0...v2.21.0)

Updates `Silk.NET.Vulkan.Extensions.KHR` from 2.16.0 to 2.21.0
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](dotnet/Silk.NET@v2.16.0...v2.21.0)

Updates `Silk.NET.Vulkan` from 2.16.0 to 2.21.0
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](dotnet/Silk.NET@v2.16.0...v2.21.0)

---
updated-dependencies:
- dependency-name: Silk.NET.Vulkan
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: silk-net
- dependency-name: Silk.NET.Vulkan.Extensions.EXT
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: silk-net
- dependency-name: Silk.NET.Vulkan
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: silk-net
- dependency-name: Silk.NET.Vulkan.Extensions.KHR
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: silk-net
- dependency-name: Silk.NET.Vulkan
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: silk-net
...

Signed-off-by: dependabot[bot] <support@github.com>

Directory.Packages.props

@@ -39,9 +39,9 @@
     <PackageVersion Include="securifybv.ShellLink" Version="0.1.0" />
     <PackageVersion Include="shaderc.net" Version="0.1.0" />
     <PackageVersion Include="SharpZipLib" Version="1.4.2" />
-    <PackageVersion Include="Silk.NET.Vulkan" Version="2.16.0" />
-    <PackageVersion Include="Silk.NET.Vulkan.Extensions.EXT" Version="2.16.0" />
-    <PackageVersion Include="Silk.NET.Vulkan.Extensions.KHR" Version="2.16.0" />
+    <PackageVersion Include="Silk.NET.Vulkan" Version="2.21.0" />
+    <PackageVersion Include="Silk.NET.Vulkan.Extensions.EXT" Version="2.21.0" />
+    <PackageVersion Include="Silk.NET.Vulkan.Extensions.KHR" Version="2.21.0" />
     <PackageVersion Include="SixLabors.ImageSharp" Version="2.1.8" />
     <PackageVersion Include="SixLabors.ImageSharp.Drawing" Version="1.0.0" />
     <PackageVersion Include="SPB" Version="0.0.4-build32" />