Oidc(ReactiveOAuth2)UserService uses userNameAttributeName when user info endpoint not invoked

[rwinch]

The property UserInfoEndpoint.userNameAttributeName is used to resolve the username from the even when the user info endpoint is not invoked (imperative / reactive).

The attribute should either:

1. Only be used when the user info endpoint is invoked (when not invoked it would always use IdTokenClaimNames.SUB). This would be a breaking change or
2. Be relocated to a place that is not specific to the user info endpoint. In this case, we could address this at the same time as the deprecation in Add SpEL support for nested username extraction in OAuth2 user info #16857

Note that this decision blocks gh-16857 since the usernameExpression might need moved off of the UserInfoEndpoint.

cc @jgrandja