Support multi site using symfony runtime

[eerison]

I'm creating this issue to check if it's necessary or not to use symfony runtime for multi site

In symfony 5.3 was introduced a new runtime, and the current implementation for multi site changes the front controller. Probably to keep this functionality working in the next major we need to implement a runtime for this.

[eerison]

~this ticket is blocked by: #1417~

[eerison]

It's working with symfony 4 and 5

then I guess it's fine for now, maybe we can provide some symfony runtime later!