DB: Add Serbia and Montenegro to coverage area

[jbruechert]

these also became recently available

[1Maxnet1]

Looks good to me 👍

[vkrause]

Probably worth using --bounding-box 36.5 -9 71 40 when regenerating the polygons, to clip away the French and Dutch oversea territories. Those produce a lot of noise and massively expand the bounding box of the covered area, without actually having coverage. Missed that last time, I just noticed it while looking at the current coverage geometry for debugging something else.