Fix content root discovery with slnx

[dmytro-pryvedeniuk]

This PR fixes the issue with content root discovery for the tests using Alba + minimal api + slnx (like polecat).

1. For net8 and net9 Microsoft.AspNetCore.Mvc.Testing packages are updated (see [release/8.0] Add .slnx fallback for TestHost content root discovery dotnet/aspnetcore#65000 and [release/9.0] Add .slnx fallback for TestHost content root discovery dotnet/aspnetcore#64953). For net10 the issue is fixed - polecat can just update to the recent version.
2. Fixed all warnings. Including vulnerability warnings like warning NU1903: Package 'System.Text.Json' 8.0.4 has a known high severity vulnerability, https://github.com/advisories/GHSA-8g4q-xg66-9fp4 [/home/runner/work/alba/alba/src/Alba.sln] by pinning Microsoft.Extensions.Hosting and Microsoft.Identity.Web in the sample/test projects.
3. Used net8 and net9 for Alba.Testing to be aligned with Alba and updated the existing tests.
4. Updated the tests added recently in Avoid extra cmd arguments causing issues with Wolverine-based systems #220 to avoid changes in the documentation sample.