Prepare plugin for Sylius 2.3 and Symfony 8

[rust-le]

Q	A
Branch?	2.2
Bug fix?	no
New feature?	yes
Related tickets	-

Adds support for Sylius 2.3 and Symfony 8.

• Service definitions (plugin and Behat) converted from XML to PHP, as Symfony 8 no longer loads XML configs
• The credit memo Gaufrette filesystem is registered as plain services instead of through knp_gaufrette bundle config, since knplabs/knp-gaufrette-bundle is no longer part of Sylius 2.3; knplabs/gaufrette is now required directly
• Request::get() replaced with $request->attributes->get() and ConversionException::conversionFailed() replaced with a plain ConversionException, both removed in Symfony 8 / DBAL 4
• Behat configuration migrated from YAML to PHP (behat.dist.php) and step annotations converted to attributes, so the same setup works on Behat 3 and Behat 4
• The test application loads Sylius' Behat services from the PHP config when available (Sylius 2.3), falling back to XML on older versions
• Dependencies allow Symfony 8 and Behat 4; the abandoned friends-of-behat/mink and friends-of-behat/suite-settings-extension are dropped, dmore/behat-chrome-extension is swapped for sylius-labs/behat-chrome-extension
• Build matrix covers Sylius ~2.2.0 and ~2.3.0 on PHP 8.3/8.4 with Symfony 6.4 and 7.4, plus Sylius ~2.3.0 on Symfony 8 (PHP 8.4)

Behat 4 is pinned to 4.0.0-alpha1 next to ^3.22, the same as in Sylius 2.3.0: the stable 4.0.0 rejects step patterns that capture more arguments than the step method accepts, which some of Sylius' own step definitions still do. friends-of-behat/mink-extension has no stable Behat 4 compatible release yet, so it is pinned to 3.0.0-alpha1 next to ^2.7.