Drop support for DBAL 3

[derrabus]

This PR removes all compatibility code for DBAL 3. I'm flagging it as a draft for now because I believe that we can backport a few things.

[greg0ire]

There is code here that could be simplified I think:

orm/src/Mapping/ClassMetadataFactory.php

Lines 627 to 660 in 3907872

	// DBAL 3
	if (method_exists($platform, 'getIdentitySequenceName')) {
	$nonIdentityDefaultStrategy[Platforms\PostgreSQLPlatform::class] = ClassMetadata::GENERATOR_TYPE_SEQUENCE;
	}
	foreach ($nonIdentityDefaultStrategy as $platformFamily => $strategy) {
	if (is_a($platform, $platformFamily)) {
	if ($platform instanceof Platforms\PostgreSQLPlatform) {
	Deprecation::trigger(
	'doctrine/orm',
	'https://github.com/doctrine/orm/issues/8893',
	<<<'DEPRECATION'
	Relying on non-optimal defaults for ID generation is deprecated, and IDENTITY
	results in SERIAL, which is not recommended.
	Instead, configure identifier generation strategies explicitly through
	configuration.
	We currently recommend "SEQUENCE" for "%s", when using DBAL 3,
	and "IDENTITY" when using DBAL 4,
	so you should use probably use the following configuration before upgrading to DBAL 4,
	and remove it after deploying that upgrade:
	$configuration->setIdentityGenerationPreferences([
	"%s" => ClassMetadata::GENERATOR_TYPE_SEQUENCE,
	]);
	DEPRECATION,
	$platformFamily,
	$platformFamily,
	);
	}
	return $strategy;
	}
	}