Remove PHP version suffixes on "Missing Required Extensions"; Mention dnf

Summary:
Explicitly mentioning PHP5 packages is outdated and confusing.
Also, mentioning dnf together with yum (apparently yum is still in use!). Good for yum users.

Closes T15352

Test Plan: Applied this change and strings on the "Missing Required Extensions" page rendered as expected.

Reviewers: O1 Blessed Committers, valerio.bozzolan, jgleeson

Reviewed By: O1 Blessed Committers, valerio.bozzolan, jgleeson

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

Maniphest Tasks: T15352

Differential Revision: https://we.phorge.it/D25203