Rename 'ReturnUrl' to 'returnUrl' for forms auth redirection

(cherry picked from commit 812712e2843a738054c065a6d5c1b7c81c5f8e7b)

src/Radarr.Http/Authentication/AuthenticationBuilderExtensions.cs

@@ -40,6 +40,7 @@ namespace Radarr.Http.Authentication
                     options.LoginPath = "/login";
                     options.ExpireTimeSpan = TimeSpan.FromDays(7);
                     options.SlidingExpiration = true;
+                    options.ReturnUrlParameter = "returnUrl";
                 })
                 .AddApiKey("API", options =>
                 {