The Symfony 5 stubs are not used anymore when bumping to Symfony 6

[VincentLanglet]

Hi @seferov,

Since #227 from @chalasr, Symfony 6 is supported ; but that's not really true.

Here https://github.com/psalm/psalm-plugin-symfony/blob/master/src/Plugin.php#L74 is loading

__DIR__.'/Stubs/'.Kernel::MAJOR_VERSION

which doesn't exist for Symfony 6.

But all the stubs in the Symfony 5 folder are still valid for Symfony 6.

What's the best strategy ? Duplicating the '5' older into a '6' folder ?