Update PagesDeployment deletion strategy to use `delete_at`

Currently, the deletion of deprecated pages deployments is based on the idea that each project must have only 1 active PagesDeployment in a given moment. With the work to introduce Gitlab Pages Multiple Versions, a project now will have to support multiple PagesDeployment at the same time.

For this reason, PagesDeployment#deleted_at field was introduced to enable us to delete deprecated jobs by setting their deleted_at to the minimal timestamp when it's safe to delete them from the database.

Now, we're replacing the DestroyPagesDeploymentsWorker by Pages::DeprecatedDeploymentsDeleteCronWorker which will delete all depreacted PagesDeployment records, in other words, all PagesDeploymen records that where deleted_at is not in the future.