fix(db-update): resolve database updates not working due to inaccessible admin-ajax.php

[ravinderk]

User Story

Background updates currently trigger with an external admin-ajax.php request on the same server, but we found that some servers disable admin-ajax.php to harden security.

Because of this, site admins are stuck and unable to update the Database without asking their web host to disable admin-ajax.php security rules.

We do not want plugin users to compromise security for DB upgrade, so we need a fallback to account for servers preventing external calls to admin-ajax.php.

##Acceptance criteria

• trigger updates from internal admin-ajax.php request as a fallback for the ajax inaccessible error.
• display an alert to admin to not navigate away from the page during the update.
• add additional explanation to the DB Updates documentation at https://givewp.com/documentation/core/frequent-troubleshooting-issues/handling-database-update-issues/
• confirm that non-affected (the other sites where it is working now) are not adversely affected by the changes.