fix: limit tests to versions they work in

Author: egil

test/TimeProviderExtensions.Tests/ManualTimeProviderTests.cs

@@ -23,7 +23,7 @@ public void SetUtcNow_updates_UtcNow()
 
         sut.GetUtcNow().Should().Be(startTime + TimeSpan.FromTicks(1));
     }
-
+#if NET8_0_OR_GREATER
     [Fact]
     public async Task Delay_callbacks_runs_synchronously()
     {
@@ -74,4 +74,5 @@ await Task.Delay(TimeSpan.FromDays(1))
             }
         }
     }
+#endif
 }