tests: Bluetooth: Mesh: make ext adv default in bsim mesh

Make extended advertiser as deafult advertiser for
general and persistent bsim BLE Mesh tests.
Leave legacy advertiser as default advertiser for
low latency bsim BLE Mesh tests.

The legacy advertiser provides quite poor performance.
Even more, it is not fully compatible with specification
since might spur more frames occasionally then requested.
This is fundamental issue since it cannot stop an advertisement
in exact time. The extended advertiser doesn't have such problem.
However, it is not well tested yet. Since extended advertiser is
not experimental anymore in both controller and host this is
the right time to make ext advertiser as a default in Mesh either.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>