Update title and description of scheduled job

Description

The scheduled job now can delete partial form submissions and the submissions of deleted forms.

So, we should to update the job's title and description for clarity.

1. Install Forms 4.12+.
2. Open Admin mode > Admin tab > Scheduled jobs
3. Open "Partial Form Submissions Removal" title.

Expected: Title & description should be updated.

Actual: Title "Partial Form Submissions Removal" and description: "Remove all partial submission data that live for more than a specific days (default is 30)"

The new title is: Form Submissions Cleanup

And the new description is: All incomplete form submission data are removed after a specified number of days. (The default number of days is 30.) In addition, all submissions of forms that are no longer available are also deleted as well.