Identity: Message for invalid username does not reflect actual permitted characters.

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

The message for an invalid username is `Username '{0}' is invalid, can only contain letters or digits`. https://github.com/dotnet/aspnetcore/blob/main/src/Identity/Extensions.Core/src/Resources.resx#L164-L167

However, this message is produced when the username contains any characters not included in `AllowedUserNameCharacters`, a value which defaults to alphanumeric _plus_ `-._@+`.

### Expected Behavior

At the very least I'd expect the error message reflect what's actually allowed by the defaults. Bonus points if it reflected what's configured by the developer, but this would be quite difficult due to the way that the allowed characters are configured.

### Steps To Reproduce

Repro is trivial:
- In an AspNetCore project w/ Identity, call `UserManager.SetUserNameAsync(user, "userName!")`
- Observe that the error states the username can only contain letters and numbers.
- Change the username to `userName@example.com`, observe that no error occurs despite containing characters other than letters and numbers as stated by the validation message.

### Exceptions (if any)

_No response_

### .NET Version

6.0.101

### Anything else?

_No response_