manifest: Update bsim to version v2.6

Main changes since v2.5:
* ext_2G4_phy_v1: Now supports DISCONNECTS during abort re-evaluations
* ext_2G4_phy_v1: Also support bitrates multiple of 250Kbps and 333Kbps

Note: Like before, bsim remains fully backwards compatible

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>