Remove await Task.FromResult() and unnecessary async modifiers (#16535)

* Remove await Task.FromResult and unnecessary async modifiers

* Remove usage of await Task.WhenAll() to avoid deadlocks

* Fix code formatting

---------

Co-authored-by: Andy Butland <abutland73@gmail.com>

tests/Umbraco.Tests.Integration/ManagementApi/ManagementApiTest.cs

@@ -31,11 +31,12 @@ public abstract class ManagementApiTest<T> : UmbracoTestServerTestBase
     where T : ManagementApiControllerBase
 {
     [SetUp]
-    public async Task Setup()
+    public Task Setup()
     {
         Client.DefaultRequestHeaders
             .Accept
             .Add(new MediaTypeWithQualityHeaderValue(MediaTypeNames.Application.Json));
+        return Task.CompletedTask;
     }
 
     protected override void CustomTestAuthSetup(IServiceCollection services)