[weather dataengine] Fix BBC provider to adapt to change RSS feed

Summary:
The urls of the BBC weather feeds seem to have changed, resulting in the
dataengine to use no longer existing urls.

I could not find the change documented somewhere and also do not know about
a contact person with BBC, so done based on examples on website, trial &
error.
There are some data issues visible in the new data feeds, so chance is the
feeds might see more rework and might break things again. But without any
contract or formal agreement we can just play catch-up.

BUG: 392510

Test Plan:
Existing configured weather applets using a location from a BBC weather
service work again.
Newly configured weather applets using a BBC weather service also work.

Reviewers: Plasma, jriddell, cfeck

Reviewed By: cfeck

Subscribers: cfeck, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11808