Bump BenchmarkDotNet from 0.13.12 to 0.15.6

---
updated-dependencies:
- dependency-name: BenchmarkDotNet
  dependency-version: 0.15.6
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Benchmarks/Schema.NET.Benchmarks/Schema.NET.Benchmarks.csproj

@@ -12,7 +12,7 @@
   </ItemGroup>
 
   <ItemGroup Label="Package References">
-    <PackageReference Include="BenchmarkDotNet" Version="0.13.12" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.15.6" />
   </ItemGroup>
 
 </Project>