Add PHP version requirement header to readme.txt

[netweb]

The plugin directory now supports declaring the minimum required version of PHP for plugins

See ​#WP40934 and ​#meta2952

[netweb]

This should be backported to appropriate branches and pushed to the plugin repo at plugins.svn.w.org :boom:

[netweb]

Here's an example plugin already using it ​https://wordpress.org/plugins/find-posts-using-attachment/

[DJPaul]

Feel free to do this for trunk (both dev/trunk and plugins/trunk), but no point doing it for older branches (we don't have branches on plugins.svn; and I would assume the parser reads trunk/readme.txt) as there's little value.

[djpaul]

In 11694:

Add PHP version requirement header to readme.txt

Fixes #7589

Props netweb