remote-mediawiki doc: don't hardcode Debian PHP versions

Change the hardcoded version 5 PHP versions to the version-agnostic packages. Currently Debian stable's version is 7.3, and there's a php7.3, php7.3-cli etc. package available (but no php5-*). The corresponding version-less package is a dependency package which depends on whatever the current stable version is. By not hardcoding the version these instructions won't be out of date when the next Debian/Ubuntu release happens. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>