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](https://lab.razor.fyi/#4wrg4gooyk8vSszVSy4WciotzsxLVwiuLC5JzbXm4iooTcrJTFZIzkksLlZwVqjmUlBQUIAKppal5pUoOCaXZObnKfjnuYK41ly1XsxFpXkJjBWMAA)

```csharp
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`.

Start by reproducing diagnostic CS8618 with the linked .NET Lab example, then trace where the diagnostic message is generated for events. Update the advice so it does not suggest `required` for events, and add or adjust coverage for both static and non-static events so the expected wording is verified.

Written by the indexing model from the issue text.