Deprecate fixture_loader option

[franmomu]

ContainerAwareLoader class is deprecated in Symfony 6.4 (and removed in 7.0) and looking at the usages I bumped into fixture_loader option.

This option was introduced long time ago in 5a1208c and it was used to instantiate that fixture loader in LoadDataFixturesDoctrineODMCommand.

Right now this option apparently is only used to set this parameter:

DoctrineMongoDBBundle/DependencyInjection/DoctrineMongoDBExtension.php

Lines 98 to 99 in e8bc048

	// set the fixtures loader
	$container->setParameter('doctrine_mongodb.odm.fixture_loader', $config['fixture_loader']);

But we don't use this parameter since 3.6 in #529 (correction: in 3.6 was still used but in a deprecated path that was removed in 4.0)

I decided to apart from deprecating the option, to remove the parameter (I can re-add it) which I guess/hope no one is using it (they shouldn't).

[malarzm]

I guess/hope no one is using it (they shouldn't).

Let's add our reasoning to UPDATE-4.7md file and I'm willing to take a chance on removing this now 👍

[malarzm]

Thanks @franmomu!