Bump Silk.NET.BuildTools from 2.20.0 to 2.21.0 (#2159)

Bumps [Silk.NET.BuildTools](https://github.com/dotnet/Silk.NET) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/dotnet/Silk.NET/releases)
- [Commits](v2.20.0...v2.21.0)

---
updated-dependencies:
- dependency-name: Silk.NET.BuildTools
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

tests/SilkTouch/Silk.NET.SilkTouch.UnitTests.csproj

@@ -12,7 +12,7 @@
 
   <ItemGroup>
     <PackageReference Update="NUnit" Version="4.1.0" />
-    <PackageReference Include="Silk.NET.BuildTools" Version="2.20.0" />
+    <PackageReference Include="Silk.NET.BuildTools" Version="2.21.0" />
     <PackageReference Include="System.Linq.Async" Version="6.0.1" />
     <PackageReference Include="Verify.DiffPlex" Version="3.0.0" />
     <PackageReference Include="Verify.NUnit" Version="23.7.1" />