#422 - Update datetime to use Java8 time rather than old Date object

As described in the noteblock under https://quarkus.io/guides/rest-json#creating-the-maven-project, Java 8 time library support has already been included. This uses ISO-8601 standards, of which RFC-3339 is a subset of. This support produces an output like 2022-02-19T21:07:58Z which is in spec.