Make label of unsupported tablet orientation change clearer

In !2400 (merged) , the greyed-out combo-box label of an unsupported change of tablet orientation was changed from "Default" to "Unsupported".

That sparked the question in that MR if this could be misconstrued as "Unsupported ... by unfinished Plasma?!" instead of the correct "Unsupported by hardware" by a non-technical user. While the new label "Not Applicable" might not be the optimum label as well, in the discussion it was regarded as "an improvement" in comparison to "Unsupported".