Call getUserIdentifier instead of getUsername on Symfony 6

[imjoehaines]

Goal

We automatically detect the logged in user's ID, which currently uses getUsername to fetch an identifier:

bugsnag-symfony/DependencyInjection/ClientFactory.php

Lines 391 to 393 in d094e0e

	if ($user instanceof UserInterface) {
	return ['id' => $user->getUsername()];
	}

Symfony 5.3 deprecated getUsername and introduced getUserIdentifier to replace it. Symfony 6.0 has subsequently removed getUsername, leaving only getUserIdentifier. This causes a crash as we're calling a method that may not exist (older code bases may still implement getUsername in addition to getUserIdentifier)

See #141

Testing

I've added a Maze Runner test for our user detection logic on Symfony 4 and 6. This covers both the getUsername branch, as well as the getUserIdentifier branch

I'll add the same test setup for Symfony 2 & 5 separately, to avoid making this PR too huge

[kattrali]

looks good - one query for futureproofing