Drop compatibility with PHP 5.6, 7.0, 7.1

[enricobattocchi]

Tentative list:

• Remove the notification introduced in DUPP-668 Use WHIP to deprecate PHP < 7.2 #19262 + remove the WHIP dependency
• Remove the WHIP dependency in Premium (see https://github.com/Yoast/wordpress-seo-premium/pull/3765 and https://github.com/Yoast/wordpress-seo-premium/pull/3803)
• Remove from GitHub actions lining, unit testing and integration testing for:

Tasks

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1. Drop compatibility with PHP 5.6, 7.0 and 7.1 #19948
   changelog: other +1
2. https://github.com/Yoast/wordpress-seo-premium/pull/3864
3. https://github.com/Yoast/wordpress-seo-local/pull/2404
4. https://github.com/Yoast/wpseo-woocommerce/pull/890
5. https://github.com/Yoast/wpseo-news/pull/792
6. https://github.com/Yoast/wpseo-video/pull/997
7. Drop compatibility with PHP 5.6, 7.0 and 7.1 duplicate-post#294
   changelog: other +1
8. Drop compatibility with PHP 5.6, 7.0 and 7.1 yoast-test-helper#184
   changelog: other +1
9. Drop compatibility with PHP 5.6, 7.0 and 7.1 yoast-acf-analysis#333
   changelog: other +1

• Check with DevOps which tasks can be simplified (e.g. it's not necessary to stick a certain composer version)