Support an end date to Workstream Participant Invitation status

[foolip]

We now have our first invited individual:
https://github.com/whatwg/participant-data/blob/6ed532d651106578ffa550eff2ab01d117783481/individuals.json#L3076-L3088

Per whatwg/sg#157 (comment) this invitation is valid for 36 months, so before those 36 months have passed we need to add a mechanism for an end date, at which point the participation check should start failing and say that the agreement needs to be revisited.