Update react-dropzone dependancy to v12

[jonocairns]

Feature Request

Describe the problem related to this feature request

react-dropzone now has a validate function that can be run before files are added.

Describe the solution you'd like

Update react-dropzone to 12.X, then update the prop types to allow use of the validate function.

Describe alternatives you've considered

There is no hook available before any files are added yet.

Teachability, Documentation, Adoption, Migration Strategy

Currently this repo references react-dropzone for the dropzone pass-through props. These are technically out of date. In theory there would be no documentation change required.

[cannontechnology]

Are there any plans to maintain material-ui-dropzone?