Upgrade JS deps - react-dnd

[emteknetnz]

Split from #308

Update react-dnd and react-dnd-html5-backend to ^16 in the following modules

• silverstripe/admin
• silverstripe/asset-admin
• dnadesign/silverstripe-elemental

New issues created

• Switch from react-dnd to dnd-kit #343

Acceptance criteria

• react-dnd and react-dnd-htm5-backend are upgraded to their latest major versions

[emteknetnz]

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

I'm going to recommend that we don't bother upgrading 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