chore(csharp): Bump BenchmarkDotNet from 0.14.0 to 0.15.8

---
updated-dependencies:
- dependency-name: BenchmarkDotNet
  dependency-version: 0.15.8
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

csharp/Directory.Packages.props

@@ -36,7 +36,7 @@
     <PackageVersion Include="xunit.runner.visualstudio" Version="3.1.5" />
     <PackageVersion Include="Xunit.SkippableFact" Version="1.5.23" />
     <!-- Benchmarking -->
-    <PackageVersion Include="BenchmarkDotNet" Version="0.14.0" />
+    <PackageVersion Include="BenchmarkDotNet" Version="0.15.8" />
     <!-- System -->
     <PackageVersion Include="System.Net.Http" Version="4.3.4" />
   </ItemGroup>