[applets/weather] Reduce label minimum width to accommodate narrow vertical panels

Summary:
The temperature label's minimum width was set high enough to overflow in a narrow
vertical panel. This patch reduces it, without regressing the layout elsewhere.

BUG: 413592
FIXED-IN: 5.17.3

Test Plan:

Reviewers: VDG, Plasma, kossebau

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25082