Remove required successUri subscriber requirement

[fmigneault]

The current definition of subscribers enforces the definition of a successUri:

ogcapi-processes/openapi/schemas/processes-core/subscriber.yaml

Lines 8 to 9 in 02627da

	required:
	- successUri

A possible use of omitting successUri could be a job definition that must take an action only upon failure. In such case, only the failedUri would be needed. However, the current definition enforces successUri to be provided. This forces clients to provide an unnecessary URI, which could generate callback requests, which the receiving server must silently ignore. Removing the requirement would avoid these unnecessary workarounds.