[BUG][Java] Generated code still using deprecated Jackson code in RFC3339DateFormat

[jonfreedman]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Beginning from 2.9.0 of com.fasterxml.jackson.databind ISO8601DateFormat is deprecated.
This class is used by RFC3339DateFormat.

This issue is reported against swagger-codegen but does not seem to have been moved across.

openapi-generator version

openapi-generator-maven-plugin 4.3.0 - needs to be fixed in master

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/RFC3339DateFormat.mustache

Command line used for generation

n/a - this is present in your samples

Steps to reproduce

n/a - this is present in your samples

Related issues/PRs

swagger-api/swagger-codegen#8575

Suggest a fix

FasterXML/jackson-databind#1786 discusses options, expected output is no deprecation warnings.