Bump Microsoft.AspNetCore.TestHost from 6.0.12 to 6.0.15 (#38)

Bumps [Microsoft.AspNetCore.TestHost](https://github.com/dotnet/aspnetcore) from 6.0.12 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@v6.0.12...v6.0.15)

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

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

Author: dependabot[bot]

src/Authentication/Tests/Monai.Deploy.Security.Authentication.Tests.csproj

@@ -24,7 +24,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="6.0.12" />
+    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="6.0.15" />
     <PackageReference Include="Microsoft.Extensions.Hosting" Version="6.0.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.1" />
     <PackageReference Include="xunit" Version="2.4.2" />