Use root DI extension class

[mbabker]

Q	A
Branch?	2.x
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Fixed tickets	N/A
License	MIT
Doc	N/A

Symfony 7.1 deprecates the Symfony\Component\HttpKernel\DependencyInjection\Extension class in favor of the root Symfony\Component\DependencyInjection\Extension\Extension class (the one in the HttpKernel supports the annotations cache stuff that used to be a thing in earlier Symfony versions and not needed today), so this PR updates the DI extension to clear that deprecation.

[coveralls]

coverage: 82.018%. remained the same
when pulling 29520ef on mbabker:patch-1
into fa2baa6 on liip:2.x.

[dbu]

thanks!