move rworldmap & luplot to suggests

[pfuehrlich-pik]

move rworldmap & luplot to suggests to prevent loading sp/maptools which would trigger a lengthy startup message:

The legacy packages maptools, rgdal, and rgeos, underpinning the sp package,
which was just loaded, will retire in October 2023.
Please refer to R-spatial evolution reports for details, especially
https://r-spatial.org/r/2023/05/15/evolution4.html.
It may be desirable to make the sf package available;
package maintainers should consider adding sf to Suggests:.
The sp package is now running under evolution status 2
     (status 2 uses the sf package in place of rgdal)
Please note that 'maptools' will be retired during October 2023,
plan transition at your earliest convenience (see
https://r-spatial.org/r/2023/05/15/evolution4.html and earlier blogs
for guidance);some functionality will be moved to 'sp'.
 Checking rgeos availability: FALSE