applets/digital-clock: migrate font settings

Plasma 5.26 introduced a new config entry autoFontAndSize which defaults
to true.
This means if the user customized font before (fontFamily, boldText,
italicText), in 5.26 these settings are ignored.

So we need to set autoFontAndSize=false if:
1. Any of these 3 old entries above is set.
2. No new entries introduced in 5.26 (autoFontAndSize, fontSize,
   fontWeight, fontStyleName) are set, so this is a config from 5.25.
And fontWeight should be set to 75 (Font.Bold) if boldText==true.

BUG:459372


(cherry picked from commit 6204d73e)