Prohibit map and grep in void context

 Prohibit map and grep in void context
 
 map and grep are not intended to be used as mutators, iterating
 with side-effects should be done with for or foreach loops. This
 fixes the one occurrence of the pattern, and bumps the perlcritic
 policy to severity 5 for the map and grep policies.
 
 Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
 Reviewed-by: Michael Paquier <michael@paquier.xyz>
 Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
 Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
 Discussion: https://postgr.es/m/87fsvzhhc4.fsf@wibble.ilmari.org