Add Requires plugin header information

[schlessera]

Feature description

We should add the requirements into the plugin's file header to let WordPress know what is expected:

 * Requires at least: 5.0
 * Requires PHP:      5.6

Note: I'm not sure which version of WordPress we actually do require since we stripped the AMP Stories component...

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• The plugin advertises the correct requirements to WordPress.

Implementation brief

• Add the following to the plugin file header metadata:

    * Requires at least: 5.0
    * Requires PHP:      5.6

QA testing instructions

Demo

Changelog entry

[westonruter]

@schlessera Isn't this already present? See:

amp-wp/readme.txt

Lines 4 to 9 in 4193dbc

	Requires at least: 4.9
	Tested up to: 5.4
	Stable tag: 1.5.1
	License: GPLv2 or later
	License URI: http://www.gnu.org/licenses/gpl-2.0.html
	Requires PHP: 5.6

It needs to be in the readme.txt not the plugin bootstrap file, AFAIK. See https://make.wordpress.org/plugins/2017/08/29/minimum-php-version-requirement/

[swissspidy]

Support for this in the main plugin file was added rather recently. Because in some cases WP only checks that, not the readme.

[westonruter]

Of course it does! 🤦‍♂