Generalize references to PHP 5.x Debian/Ubuntu packages

Summary:
Both Debian and Ubuntu have been shipping PHP 7.x (or now 8.x) for a while.
This patch replaces the versioned php5-foo package references with more general
php-foo package names.

Test Plan: Navigated to the documentation pages that this patch updates.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25315