Fix bug with SQL syntax error during backup restoration

## What does this MR do?
It properly escapes the table name in the `DROP TABLE IF EXISTS ...` statement used during restore of a backup.

## Are there points in the code the reviewer needs to double check?
Please check compatibility with PostgreSQL

## Why was this MR needed?
Fix an issue with backups can not be restored if MySQL is used as database for GitLab.

## What are the relevant issue numbers?
#15259 

## Screenshots (if relevant)
-

See merge request !4318