Bump Microsoft.AspNetCore.Mvc.Testing from 8.0.12 to 8.0.15 in /src

Bumps [Microsoft.AspNetCore.Mvc.Testing](https://github.com/dotnet/aspnetcore) from 8.0.12 to 8.0.15.
- [Release notes](https://github.com/dotnet/aspnetcore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@v8.0.12...v8.0.15)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.Mvc.Testing
  dependency-version: 8.0.15
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

src/Directory.Packages.props

@@ -17,7 +17,7 @@
     <PackageVersion Include="Fody" Version="6.9.1" />
     <PackageVersion Include="GitHubActionsTestLogger" Version="2.4.1" />
     <PackageVersion Include="HdrHistogram" Version="2.5.0" />
-    <PackageVersion Include="Microsoft.AspNetCore.Mvc.Testing" Version="8.0.12" />
+    <PackageVersion Include="Microsoft.AspNetCore.Mvc.Testing" Version="8.0.15" />
     <PackageVersion Include="Microsoft.AspNetCore.SignalR.Client" Version="8.0.12" />
     <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="8.0.1" />
     <PackageVersion Include="Microsoft.Extensions.DependencyModel" Version="8.0.2" />