Prevent plugin from being auto-updated in WordPress 5.5

CiviCRM cannot be updated via the WordPress auto-updater, so we should disable this.

Details of the new functionality here:

https://make.wordpress.org/core/2020/07/15/controlling-plugin-and-theme-auto-updates-ui-in-wordpress-5-5/

PR here: https://github.com/civicrm/civicrm-wordpress/pull/211