Rename RA RPC UpdateAuthorization to PerformValidation

[rolandshoemaker]

The current name is rather misleading.

[jsha]

Note: We may actually want to give PerformValidation a slightly different API, that doesn't include a keyAuthorization object. Then we can get rid of this comment and associated code:

boulder/ra/ra.go

Lines 1473 to 1482 in 465be64

	// NOTE(@cpu): Historically challenge update required the client to send
	// a JSON POST body that included a computed KeyAuthorization. The RA would
	// check this provided authorization against its own computation of the key
	// authorization and err if they did not match. New ACME specification does
	// not require this - the client does not need to send the key authorization.
	// To support this for ACMEv2 we only enforce the provided key authorization
	// matches expected if the update included it.
	if response.ProvidedKeyAuthorization != "" && expectedKeyAuthorization != response.ProvidedKeyAuthorization {
	return core.Authorization{}, berrors.MalformedError("provided key authorization was incorrect")
	}