Chore: Remove public vars in setting package (#81018)

Removes the public variable setting.SecretKey plus some other ones. 
Introduces some new functions for creating setting.Cfg.

pkg/middleware/recovery.go

@@ -152,7 +152,7 @@ func Recovery(cfg *setting.Cfg, license licensing.Licensing) web.Middleware {
 						Assets    *dtos.EntryPointAssets
 					}{"Server Error", "Grafana", cfg.AppSubURL, cfg.DefaultTheme, "", assets}
 
-					if setting.Env == setting.Dev {
+					if cfg.Env == setting.Dev {
 						if err, ok := r.(error); ok {
 							data.Title = err.Error()
 						}