Update PHP versions in install guide: Remove 5, Add 8.

Summary: Ref T15737. Ref T15554. Ref T15064. Ref T15047.

Test Plan: Generate diviner, read section.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15737, T15554, T15064, T15047

Differential Revision: https://we.phorge.it/D25537