add color gray for 255 value (>2) to GNSS antena status's valueColors

closes #277 (closed)

UPDATE Parameters set valueColorsB='<=0:#00FF00|<=1:#FFFF00|<=2:#FF0000|2<:#CCCCCC', valueColorsW='<=0:#00CC00|<=1:#DDDD00|<=2:#FF0000|2<:#CCCCCC' WHERE param='GNSS antenna status';