Bump Microsoft.AspNetCore.TestHost from 6.0.9 to 6.0.15 (#2054)

Bumps [Microsoft.AspNetCore.TestHost](https://github.com/dotnet/aspnetcore) from 6.0.9 to 6.0.15.
- [Release notes](https://github.com/dotnet/aspnetcore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@v6.0.9...v6.0.15)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.TestHost
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

test/RestSharp.Tests.Integrated/RestSharp.Tests.Integrated.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
         <Nullable>enable</Nullable>
         <TargetFrameworks>net6.0;net7.0</TargetFrameworks>
@@ -16,7 +16,7 @@
     </ItemGroup>
     <ItemGroup>
         <PackageReference Include="HttpTracer" Version="2.1.1" />
-        <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="6.0.9" />
+        <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="6.0.15" />
         <PackageReference Include="Polly" Version="7.2.3" />
         <PackageReference Include="Xunit.Extensions.Logging" Version="1.1.0" />
     </ItemGroup>