Remove Web Auth method embedded in Authentication API client

[Widcket]

Changes

⚠️ THIS PR CONTAINS BREAKING CHANGES

The Authentication API client has a method webAuth(withConnection:) that actually uses Web Auth under the hood. Its implementation is:

func webAuth(withConnection connection: String) -> WebAuth {
        let webAuth = Auth0WebAuth(clientId: self.clientId, url: self.url, telemetry: self.telemetry)
        return webAuth
            .logging(enabled: self.logger != nil)
            .connection(connection)
    }

Besides being unnecessary, it is confusing, so this PR removes it in favor of just using Auth0.webAuth().connection().

Testing

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed