Rename onCheckoutBeforeProcessing to onCheckoutValidationBeforeProcessing

[nerrad]

This idea came up as I considered the problem outlined in #3980 when I realized the onCheckoutBeforeProcessing is tightly coupled to the validation when the checkout has been submitted (as a quick way to alert the user there is something missing/wrong before submitting to the server).

Given its use-case, I suggested that it would make sense to change the name of this event to something like onCheckoutValidationBeforeProcessing to more clearly articulate its purpose and behaviour.

For implementation, we will need to make sure we keep back compat for current implementation of this hook outside of our own code (and throw a deprecation notice for when onCheckoutBeforeProcessing is called instead of the new function. Not sure we'd ever remove the deprecated event (or at least far in the future) but at least we can stop promoting it (in docs) and elsewhere it's outlined.