net: sockets: Move offloading out of experimental

Socket offloading has been in the tree for a while and improved a lot
over time (from a simple define-based API override to a complex
vtable-based solution, supporting mutliple offloaded interfaces). As the
feature is heavily used by certain vendors (Nordic and its nRF Connect
SDK), I propose to move it out of experimental phase.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>