Add hint a new Server version isn't being offered likely due to not meeting PHP requirements

[joshtrichards]

The following leaves the admin either: (a) incorrectly reassured; or (b) confused:

Nextcloud Updater - version: v25.0.11-5-g06c1bad
Current version is 25.0.13.
No update available.
Nothing to do.

Two situations this is a problem in:

• when current in-use Server release is EoL and PHP is too old to update to next major <-- biggest problem
• when current in-use Server release isn't EoL but PHP is too old to update to next major <-- bonus benefit of fixing the prior item