Default datetime format for REST must contain timezone

AT:

• default datetime format on backend contains timezone: yyyy-MM-dd'T'HH:mm:ss.SSSZ
• it is documented in update guide that users who want to have old date format in REST responses must use CustomJacksonDateFormatListener
• it is documented in [1] which datetime format is compatible with Camunda webapps (smth like yyyy-MM-dd'T'HH:mm:ss[.SSS[Z]] but need to be evaluated further)

So that

• we can use Webapps in timezones defferent from server timezone.

[1] https://docs.camunda.org/manual/7.7/reference/rest/overview/date-format/