[FIX] Replace references to non-existent PropTypes.function with PropTypes.func

[gustaff-weldon]

Fixes #2063

Several places in Victory reference non-existent PropTypes.function validator. This seem to be a common error, as exactly the same case is pointed out in prop-types codebase:
https://github.com/facebook/prop-types/blob/4de0644a10a554d0a556daa39f029369bc007ea5/checkPropTypes.js#L55-L57

This causes warning in console:

'Warning: Failed prop type: VictoryScatter: prop type externalEventMutations[0].callback is invalid; it must be a function, usually from the prop-types package, but received undefined.'

This PR replaces validators with PropTypes.func (https://github.com/facebook/prop-types#usage)

[gustaff-weldon]

@becca-bailey it would be great if this bugfix can be released quickly, currently we need to resort to running from fork as our CI setup fails on component render warnings

[becca-bailey]

@gustaff-weldon thanks for looking into this, I'll get this merged and released ASAP.

[becca-bailey]

We don't have permission to run the docs and CI publish scripts from a fork, which is why some CI steps were failing. We're looking into fixing this in the future, but I don't anticipate any issues with this particular PR since it is only updating prop types.

[gustaff-weldon]

@becca-bailey thanks for super quick turn-around on releasing this 🙇