Make PluginSkeleton up to date with the latest changes made in Sylius

[coldic3]

• The symfony.lock file was added to prevent loading already managed configurations.
• Symfony 6 fix: https://github.com/Sylius/PluginSkeleton/pull/387/files#diff-de1ec58376e3b692293daadf74c4531cac5eac13a4764d571a3bbeddf77ae135R14
• The Webpack configuration has been unified and all JavaScript dependencies have been replaced with sylius-ui/frontend.
• Newly created Doctrine migrations will be placed in the migrations directory. Before, it didn't work as all new migrations were placed in the vendor directory.

[GSadee]

Thank you, Kevin! 🥇