MariaDB 10.6

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create table wp_posts(post_content text);
insert into wp_posts values ('<div>hello<p>hi</p></div>'), 
('<div class="test">hello<p>hi</p></div>');

Records: 2  Duplicates: 0  Warnings: 0

select * from wp_posts;

post_content

<div>hello<p>hi</p></div>

<div class="test">hello<p>hi</p></div>

update wp_posts
set post_content = REGEXP_REPLACE(post_content, '</?div.*?>', '');

Rows matched: 2  Changed: 2  Warnings: 0

select * from wp_posts;

post_content

hello<p>hi</p>

hello<p>hi</p>