fix(buttons): allow correct ref validation in PropTypes

[austingreendev]

Description

With the new React.createRef usages, some of our old "forward-ref" props have invalid prop-types. An example error:

// When supplying React.createRef() to `buttonRef`
Warning: Failed prop type: Invalid prop `buttonRef` of type `object` 
supplied to `IconButton`, expected `function`.

Detail

This PR corrects the invalid prop-types for the react-buttons package. There are also other usages of PropTypes.func for refs in additional packages, but they are currently deprecated. I am excluding them from this PR.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• ♿ analyzed via axe and evaluated using VoiceOver
• 💂‍♂️ includes new unit tests
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage remained the same at 95.463% when pulling bb95946 on agreen/button-ref-props into c257b6c on master.

[jzempel]

I think these buttonRef propTypes just need to be removed as they are no longer valid for Button.

[dclowd9901]

Is any really the appropriate choice here? It should only be an object or a function, right?

In the other hand, if you actually want to enforce the type of an actual ref, for example a prop that is forwarded to another component's ref, you have to allow both on object shaped by React.createRef() (The preferred way to create ref in React components, see type in react code here) and function types.

Object shape: PropTypes.shape({ current: PropTypes.instanceOf(Element) }) is for refs created using React.createRef()
PropTypes.func is for callback refs as used in the above question
Which gives the following PropType definition:

forwardRef: PropTypes.oneOfType([
  PropTypes.func, 
  PropTypes.shape({ current: PropTypes.instanceOf(Element) })
])

[austingreendev]

We have avoided the shape prop-type in other packages which is why I went with any here.

We ended up removing them as they actually don't do anything in the v6+ world.