Bundles should use the modern package structure

[stof]

Bundles should have templates and config folders sibling to their src folder instead of using the Resources folder.

The easiest way to opt-in for the new structure is to extend AbstractBundle instead of Bundle as that new base class does all the overrides to the new structure (and as we don't need to support Symfony 6.0 and older, there is no valid reason to use the more complex way of doing the opt-in)

[yceruto]

I can help on this

[chr-hertel]

Thanks for driving this :)