Update the supported PHP versions in the docs

Summary: Refs T15064.

Test Plan: Generated the doc locally, see that it says what I was planning for it to say.

Reviewers: valerio.bozzolan, aklapper, O1 Blessed Committers

Reviewed By: valerio.bozzolan, O1 Blessed Committers

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25245