General: require PHP 5.3+ to be able to run Jetpack.

[jeherve]

Given that Jetpack supports WordPress’ latest version (currently 5.2) as well as one version prior, we can't benefit from the changes introduced in WordPress 5.2 yet: Jetpack currently supports WordPress 5.1, and the required PHP version set by WP 5.1 is PHP 5.2.
This PR aims to change that required PHP version for Jetpack, until WordPress 5.3 is released; at that point Jetpack can go back to following WordPress’ PHP requirements and thus require PHP 5.6.

Changes proposed in this Pull Request:

If a site runs WP 5.1 (where the PHP requirements are still PHP 5.2) and updates Jetpack:

1. We pause Jetpack (stop loading it).
2. We display an admin notice inviting folks to update PHP.
3. If WP_DEBUG is on, we log an error.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• More discussion here: p9dueE-Lz-p2

Testing instructions:

• Add the following to your wp-config.php file:
  define( 'JETPACK__MINIMUM_PHP_VERSION', '7.4' );
• Load your dashboard.
• You should see this warning and Jetpack will not be loaded:

Proposed changelog entry for your changes:

• General: Jetpack now requires PHP 5.3, and will display a notice if your site uses an older version of PHP.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: June 4, 2019.
Scheduled code freeze: May 28, 2019

Generated by 🚫 dangerJS against 29e5f51

[dereksmart]

Love these red lines :)

[gravityrail]

❤️

[lezama]

I think we could moved this inside the if ( version_compare( phpversion(), JETPACK__MINIMUM_PHP_VERSION, '<' ) ) {?