Restored missing function inside dropzone_input.js

What does this MR do?

Adds the missing closeAlertMessage function that prevented to copy/paste images into the text area for comments, issue descriptions, MRs, etc.

Are there points in the code the reviewer needs to double check?

I don't think so

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

closes #33836 (closed)