Bring back the whip message

[andizer]

This time we check against PHP version 5.6

Summary

This PR can be summarized in the following changelog entry:

• Shows a notice on both the WordPress and Yoast dashboards when the current PHP version is below 5.6.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

• Have an older version of PHP installed. (I've no clue how to achieve that).
• Or you can fake the version by changing the >= 5.6 to >=7.6 here: https://github.com/Yoast/wordpress-seo/compare/feature/oauth-connection-ui...12163-add-whip-message?expand=1#diff-e8ce9a043bfa6ca96ed62eb9f0de63b6R141
• Go to the WordPress main page and see a big error.
• Go to the Yoast dashboard and see the same error
• Go to another page and see no error.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #12163

[abotteram]

CR 👍

[Dieterrr]

Acceptance done 👍
Had to wait 4 week to get the dismissed message back but even that works as expected :)