MINOR: [C#] Bump coverlet.collector from 1.2.0 to 6.0.0 in /csharp

Bumps [coverlet.collector](https://github.com/coverlet-coverage/coverlet) from 1.2.0 to 6.0.0.
- [Release notes](https://github.com/coverlet-coverage/coverlet/releases)
- [Commits](coverlet-coverage/coverlet@v1.2.0...v6.0.0)

---
updated-dependencies:
- dependency-name: coverlet.collector
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

csharp/test/Apache.Arrow.Flight.Tests/Apache.Arrow.Flight.Tests.csproj

@@ -9,7 +9,7 @@
  <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
  <PackageReference Include="xunit" Version="2.4.0" />
  <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
- <PackageReference Include="coverlet.collector" Version="1.2.0" />
+ <PackageReference Include="coverlet.collector" Version="6.0.0" />
  </ItemGroup>
 
  <ItemGroup>