Fix SampSharp.EcsGameMode.OpenMp template startup signature and remove redundant usings - #583

The SampSharp.EcsGameMode.OpenMp template drifted from the current IEcsStartup contract after IConfiguration was added to ConfigureServices, causing generated projects to fail to compile. This updates the template to match the interface and trims template-level usings that are now provided implicitly by SampSharp.Sdk.

• Template API alignment

  • Updated template Startup to implement the current IEcsStartup signature:
    • ConfigureServices(IServiceCollection services, IConfiguration configuration)
  • Added using Microsoft.Extensions.Configuration in template Startup.cs.

• Template using cleanup (SDK implicit usings)

  • Removed explicit SampSharp.Entities* usings from:
    • Startup.cs
    • MyFirstComponent.cs
    • MyFirstSystem.cs
  • Kept only usings still required explicitly by template source.

public class Startup : IEcsStartup
{
    public void ConfigureServices(IServiceCollection services, IConfiguration configuration)
    {
    }
}