bluetooth: smp: remove experimental from BT_GATT_AUTHORIZATION_CUSTOM

Removed the experimental status from the BT_GATT_AUTHORIZATION_CUSTOM
Kconfig option used in the Bluetooth Host GATT layer. This feature
has been present in Zephyr for almost a year without any issue reports
or API modifications.

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>