$wpdb->db_version() wrongly documented

[luciole135]

The comment of db_version () function is false.
This function returns the version of MySQL, not the version of the database as specified in the source files.

<?php
/**
         * The database version number.
         *
         * @since 2.7.0
         *
         * @return null|string Null on failure, version number on success.
         */
        public function db_version() {
                if ( $this->use_mysqli ) {
                        $server_info = mysqli_get_server_info( $this->dbh );
                } else {
                        $server_info = mysql_get_server_info( $this->dbh );
                }
                return preg_replace( '/[^0-9.].*/', '', $server_info );
        }

[SergeyBiryukov]

In 36385:

Docs: Correct wpdb::db_version() description.

Props luciole135.
Fixes #35588.