Add AllowedUsernameCharacters to InvalidUserName error

[SourceCodeWhen]

Add AllowedUsernameCharacters to InvalidUserName error

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Description

Added the AllowedUsernameCharacters string to the InvalidUserName error message. This constitutes a change to the public API so would be best to double check everything is ok before merging. Maybe an overload for this method would be better for backwards compatibility? Just let me know! :)

Fixes #39375

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