fix(group_members): using ISOTime instead of time.Time for BillableUserMembership.ExpiresAt

Loïc Kreitmannrequested to merge
gitlab-community/gitlab-org/api/client-go:loickreitmann-usermembership-expiresat into main

The current implementation of the BillableUserMembership type uses *time.Time for its ExpiresAt property. This causes marshalling errors with the following message when the value is set as a date only, whan calling the GroupsService.ListMembershipsForBillableGroupMember method:

parsing time "2025-12-31" as "2006-01-02T15:04:05Z07:00": cannot parse "" as "T".

To avoid this error, I'm proposing to change this ExpiresAt property to *ISOTime.

References:

Merge request reports