CS8618 suggests using `required` on events.

Version Used: 5.3.0-2.25603.1

Similar to #78152, but for events, both static and non-static.

Steps to Reproduce:

.NET Lab

using System;

public class C {
    public event Action OnEvent;
}

Diagnostic Id:

CS8618 Non-nullable event 'OnEvent' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Expected Behavior:

The error message is:

CS8618 Non-nullable event 'OnEvent' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Actual Behavior:

The error message gives wrong advice. Adding the required modifier results in error CS0106 The modifier 'required' is not valid for this item.