Change upload license page file input to dropzone

What does this MR do and why?

Changes the license upload file input to a file dropzone

Related to #343553 (closed), designs are attached there

Screenshots or screen recordings

header	header
1. default view
2. hovering any file, drop zone currently doesn't distinguish wrong from proper files on hovering, only dropping a wrong file would show an error (see screenshot # 4)
3. successfully dropped a file with proper extension
4. dropped a file with wrong extension
5. dragging some html element on the dropzone

How to set up and validate locally

On your local setup, open http://localhost:3000/admin/license/new and try to drug-n-drop a file or click on the dropzone to trigger the open file window.

You might need to generate either a license file or just create a random *.gitlab_license file and attach it to the form. In the latter case upon form submission the file contents would be inserted into the "Enter the license key" text area with an error that there's something wrong with the code.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.