lang/php7*: adjust CONFLICTS to new versions

PHP 7.3 conflicted with versions no longer in the ports-tree. Remove this conflicts.
PHP 7.3 and 7.4 did not conflict with PHP 8.0. Add the missing conflict.

PR: 258803
Reported by: Einar Bjarni Halldórsson <einar@isnic.is>
Sponsored by: Bounce Experts