Update breakpoint to trigger core notice about outdated PHP version in Servehappy API

[flixos90]

Since WordPress [aims to raise the minimum PHP version requirement](​https://make.wordpress.org/core/2018/12/08/updating-the-minimum-php-version/) sooner than later to PHP 5.6, we should update the breakpoint in the Servehappy API to that version, so that the core notice announced for WordPress 5.1 shows up for any site below that version.

[flixos90]

4006.diff​ makes the following updates:

• Recommended PHP version now 7.3 (as released about two weeks ago)
• Minimum PHP version still actively supported now 7.2 (7.1 now only receives security updates)
• Minimum PHP version still receiving security updates now 7.1 (7.0 is no longer supported, 5.6 will no longer be supported as of December 31st)
• Minimum PHP version still acceptable by WordPress now 5.6 (due to intended version bump)

See ​http://php.net/supported-versions.php for the source for these decisions.

[dd32]

In 7990:

API: ServeHappy: Bump the required PHP versions to reflect PHP 7.3 as being the latest, and assume that it'll apply for PHP < 5.6, rather than only for PHP 5.2 as previous.

Props flixos90.
Fixes #4006.