Switch from react-dnd to dnd-kit

[emteknetnz]

Follow on from #335

We are using react-dnd 5 in admin, asset-admin and elemental. The latest version is 16.

In LinkField we implemented drag and drop using dnd-kit instead due to it being more maintained and not completely breaking silverstripe/silverstripe-linkfield#166

We should not attempt to upgrade react-dnd, and instead switch everything over to dnd-kit instead. From memory we talked about doing this at some point offline at the time we did the linkfield work

Notes

• If we can't do this for 6.0, we will aim for a minor release but if there are massive breakages for projects, we need to discuss the effects and consider if releasing it in a minor is appropriate

Acceptance criteria

• react-dnd and react-dnd-html5-backend are replaced with dnd-kit
• react-dnd and react-dnd-html5-backend dependencies are removed

New cards raised:

• CMS 6 REGRESSION - Can't move files/folders up to the root level silverstripe-asset-admin#1529
• CMS 6 REGRESSION - Moving files/folders to a new folder doesn't swap to that folder silverstripe-asset-admin#1530

PRs

Elemental

• ENH Convert ElementList into a functional component silverstripe-elemental#1297
• DEP Replace react-dnd with dnd-kit silverstripe-elemental#1298

The rest

• ENH Allow more modern syntax including null-coallescing eslint-config#29
• DEP Replace react-dnd with dnd-kit silverstripe-asset-admin#1527
• DEP Remove references to react-dnd webpack-config#79
• DEP Remove all references to react-dnd silverstripe-admin#1879
• DOC Document that javascript deps have been updated developer-docs#659