Update deprecated WPMS hooks

[ouun]

delete_blog & wpmu_new_blog are deprecated since WP 5.1. This PR follows up.

References: https://developer.wordpress.org/reference/hooks/wpmu_new_blog/ https://developer.wordpress.org/reference/hooks/delete_blog/

[stklcode]

Another decision that should at least be discussed briefly:

wp_initialize_site and wp_delete_site have both been introduced in 5.1 - Cachify currently declares compatibility with 4.4+

So we either drop support for WP <5.1 or add both hooks, maybe conditionally.

[florianbrinkmann]

So we either drop support for WP <5.1 or add both hooks, maybe conditionally.

I would vote for adding both and checking for the WordPress version.