GH-9936 Fixed datatype for brightness of the ambient light (#10065)

GH-9936 Fixed datatype for brightness of the ambient light (#10065)

The ceiling10 Yeelight lamp has 2 dimmer for brightness:
"bright" for the main light and bg_bright for the ambient light.
Both are of type dimmer and accept percentage values between 1 and 100.
This fix changes the datatype of the bg_bright property from Number
to Dimmer in the device mapping database file ceiling4, which is also
the one that is used for the ceiling10 lamp.

Accepted without sign-off under minor change exemption.