- Security: authenticator manager instead of Guard (LoginAuthenticator,
  EmailTokenAuthenticator, ApiKeyAuthenticator), UserAuthenticatorInterface
  for programmatic logins, password_hashers, PUBLIC_ACCESS, new
  UsernamePasswordToken signature, loadUserByIdentifier, getUserIdentifier.
- Role choice while impersonating: rebuild the SwitchUserToken (was a 500
  error since Symfony 5: getRoles()/SwitchUserRole no longer exist).
- Unknown email token: 401 instead of a 500 error.
- LoginListener listens to LoginSuccessEvent and uses the request session.
- Session: RequestStack::getSession() instead of the session service
  (removed in Symfony 6) in 36 classes.
- SwiftMailer replaced by symfony/mailer (MAILER_DSN instead of MAILER_URL).
  DeferredMailer sends after the response and logs the errors, like the
  former memory spool.
- twig/extensions removed: the truncate filter is ported in App\Twig\TextExtension.
- Tests: AuthenticationTest (remember me, API key, email token, role choice
  while impersonating), email assertions, DeferredMailer and truncate unit tests.