Modification of the endpoint workflows?expand=manifest so that the timestamps are in UTC to resolve the 2-hour discrepancy in SquashTM.

Issue is relate to this https://gitlab.com/henixdevelopment/squash/squash-tm/core/squashtest-tm-staging/-/issues/2603

The orchestrator's API http://127.0.0.1:7775/workflows?expand=manifest returns the date in CEST (Paris), but there is no timezone information included, so the ObjectMapper in Java assumes it is in UTC, adding 2 hours to the timestamp.