String announcement for Maps

Hi!

There will be 3 new strings for Maps:

These relate to the migration to use an AdwAboutDialog, showing headers
in the "Legal" section for data providers:

- "Map Data Provider"

Section header for map data copyright (mentioning OpenStreetMap and
contributors.

- "Map Tile Provider"

Section header for showing contribution for map tile provider, the
service serving pre-rendered map tiles.

- "Search Provider"

Section header for showing contribution for the search provider
service.

See also this MR:

https://gitlab.gnome.org/GNOME/gnome-maps/-/merge_requests/240

Cheers!

//Marcus