[MAINTAINER] www/mod_perl2: fix for apache21 and higher

• variant of the fix in ports/90312 (can be closed, too)

• warn if an apache newer than apache20 is used

	  (maintainer can't test)

• remove warning about API change, it's been a year

PR: ports/90655
Submitted by: Lars Eggert <lars.eggert@gmx.net>