Bumped the Symfony version used to the recently released 5.3.

I had to add the symfony/runtime requirement (it's also present in https://github.com/symfony/website-skeleton/blob/5.3/composer.json#L30).

There are other differences between these two skeletons:

1. sensio/framework-extra-bundle is using different major: https://github.com/symfony/website-skeleton/blob/5.3/composer.json#L30 (we have `"sensio/framework-extra-bundle": "^5.6.1")
2. We still have "doctrine/doctrine-bundle": "~2.3.2" - added in IBX-460: Locked doctrine/doctrine-bundle to 2.3.x to avoid issues #3 , not sure if it's still needed.

Currently this PR fails to install with:

!!  In CheckExceptionOnInvalidReferenceBehaviorPass.php line 86:
!!
!!    The service "ezpublish_rest.security.authentication.listener.session.ezpubl
!!    ish_front" has a dependency on a non-existent service "session.storage". Di
!!    d you mean this: "session.storage.factory.native"?

which seems related to symfony/symfony#40048

Checklist:

• Provided PR description.
• Tested the solution manually.
• Checked that target branch is set correctly.
• Asked for a review (ping @ibexa/engineering).