Bump ServeHappy PHP recommendation

[Clorith]

After some considerations and discussions across a few teams, there's an agreement to start rolling out an increase of upgrade recommendations with the ServeHappy widget.

To give time to prepare, we'd like to do this on August 20th (roughly one week from this tickets creation), this gives us ample time to communicate the intent now that we're past the information gathering part of the process.

Given that a large portion of users are still on PHP 5.6, jumping straight to 7.2 is a little bit premature. The idea is therefore to update the recommendation to 7.0 for this first new update.

This means that any site running PHP 5.6 or lower would see the PHP update recommendation widget.

This ticket is merely a courtesy to keep things tracked, and serves as a reminder to do the bump on 2019-08-20

[SergeyBiryukov]

In 9112:

API: ServeHappy: Bump the minimum acceptable PHP version to 7.0.

Props Clorith.
Fixes #4671.