NotImplemented status code

[AdrienSluicebox]

The openapi defines Not implemented responses for http status code 501:

pact-openapi/pact-openapi-2.1.0.yaml

Lines 178 to 183 in baee586

	'501':
	description: Not implemented
	content:
	application/json:
	schema:
	$ref: '#/components/schemas/ErrorResponse'

However the spec says that NotImplemented responses should have status code 400: https://wbcsd.github.io/data-exchange-protocol/v2/#business-cases-async-events-1-cases :

The events endpoint responds with HTTP error code 400 and with a body with error code NotImplemented