wpdb::_insert_replace_helper() incorrectly quotes prepare placeholders

[duck_]

wpdb::_insert_replace_helper() implodes $formatted_fields with "','" this produces something like:

INSERT INTO `wp_terms` (`name`,`slug`,`term_group`) VALUES ('%s','%s','%d')

and not

INSERT INTO `wp_terms` (`name`,`slug`,`term_group`) VALUES (%s,%s,%d)

This is inherited from wpdb::insert(), see [10724], so marking version as 2.8 rather than 3.0.

In [19692]:

Stop quotes being incorrectly added to prepare placeholders in wpdb::_insert_replace_helper(). Fixes #19016.