Minimal api changes

[deepchoudhery]

Addressing #1937

• using routes.MapGroup instead of routes.Map(Get/Put/Post etc.)
• adding Microsoft.AspNetCore.Http.HttpResults to usings.
• changing nullable DbContext set from public DbSet<Model>? DbSet { get; set; } --> public DbSet<Model> DbSet { get; set; } = default! for parity with scaffolding templates.
• addressing Minimal API Scaffold generates a wrong Created location #1963
• addressing Scaffolding minimal api should use lower case parameter names for generated lambdas #1954

[deepchoudhery]

@mkArtakMSFT anyone from the aspnetcore you reckon can review these changes?