[JACKSON] ISO8601DateFormat deprecation warnings

[shexbeer]

Description

Beginning from 2.9.0 of com.fasterxml.jackson.databind ISO8601DateFormat is deprecated.
This class is used by RFC3339DateFormat which gets generated by swagger-codegen maven plugin.

Swagger-codegen version

2.3.1

Suggest a fix/enhancement

It is recommended to use com.fasterxml.jackson.databind.util.StdDateFormat instead.

[jonfreedman]

See FasterXML/jackson-databind#1786