Update links to Dropzone library

Problem/Motivation

The dropzonejs module declares the DropzoneJS library license URL as
https://github.com/enyo/dropzone/blob/master/LICENSE in
dropzonejs.libraries.yml. The upstream DropzoneJS project has moved from
enyo/dropzone to dropzone/dropzone and renamed its default branch
from master to main. As a result, the old license URL now returns a
Redirect

Steps to reproduce

1. Install the dropzonejs module.
2. Open dropzonejs.libraries.yml (or view the library license link).
3. Follow the license URL https://github.com/enyo/dropzone/blob/master/LICENSE.
4. Observe that GitHub returns to the new URL, lets just alter it with the new url instead.

Proposed resolution

Update the license URL in dropzonejs.libraries.yml to the new upstream location:

license:
  name: MIT
-    url: https://github.com/enyo/dropzone/blob/master/LICENSE
+    url: https://github.com/dropzone/dropzone/blob/main/LICENSE
  gpl-compatible: true

Remaining tasks

• Review and commit the patch.

User interface changes

None (the displayed license link now resolves correctly instead of 404).

API changes

None.

Data model changes

None.