Use prefix instead of parsing our own

[JoryHogeveen]

Enhance compatibility with other plugins.

Related GitHub issue(s)

Fixes #7181

Changelog text for these changes

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).

[what-the-diff]

PR Summary

• Updated variable in pick.php file
  In our code, specifically in a file called pick.php, we've replaced a piece of code ($wpdb->base_prefix) with a different one ($wpdb->prefix). This might seem like a small change, but it's crucial for keeping the system running smoothly and effectively.

[sc0ttkclark]

There was a reason this was in place, I need to go figure out why that was and how to continue supporting that (if it's still a valid reason).

[sc0ttkclark]

Walking backwards from most recent to the original introduction of this usage:

• We switched to $wpdb->base_prefix from $wpdb->prefix in Use base_prefix instead of prefix #2393 to fix subsite issues
• We introduced the $wpdb->prefix usage instead of hardcoding wp_ in Don't assume wp_ is the prefix #2385 Non wp_ table prefix -> user relation by role broken #2306
• We fixed usage so subsites got the current blog ID added to the prefix in Multiple select relating to Users is empty when Relationship is limited by Role #1415
• We introduced the ability to limit by user roles in ab1370d

Given that, I think your PR looks good -- EXCEPT -- this is not what WP_User uses (as of WP 4.9+) and that's what we need to follow instead here.

	/**
	 * Sets the site to operate on. Defaults to the current site.
	 *
	 * @since 4.9.0
	 *
	 * @global wpdb $wpdb WordPress database abstraction object.
	 *
	 * @param int $site_id Site ID to initialize user capabilities for. Default is the current site.
	 */
	public function for_site( $site_id = '' ) {
		global $wpdb;

		if ( ! empty( $site_id ) ) {
			$this->site_id = absint( $site_id );
		} else {
			$this->site_id = get_current_blog_id();
		}

		$this->cap_key = $wpdb->get_blog_prefix( $this->site_id ) . 'capabilities';

		$this->caps = $this->get_caps_data();

		$this->get_role_caps();
	}

So the solution here needs to actually be calling $wpdb->get_blog_prefix() instead.

[sc0ttkclark]

Just did more digging into the wpdb class and it looks like when switching blogs, it accounts for everything automatically so the code that WP_User is using isn't necessary since our context doesn't need to be able to check by site ID -- only the current site context. We can merge this as-is.