PR #25406 - OpenAPITools/openapi-generator

Adds a useJackson3 option to the jaxrs-spec generator. Jackson 2 stays the default.

useJackson3 option + cliOption, jacksonPackage property, JsonDeserialize import mapping (moved to tools.jackson.databind in Jackson 3)
spec/pom.mustache: {{jacksonPackage}} groupIds for the JAX-RS providers, jackson-datatype-jsr310 only for Jackson 2 (built into databind in Jackson 3), Jackson 3 version 3.1.2 (aligned with the CXF generator)
pojo.mustache: Shape.POJO instead of Shape.OBJECT for Jackson 3 when additional properties are used
Test testUseJackson3 in JavaJAXRSSpecServerCodegenTest
Docs regenerated. jaxrs-spec samples regenerated: no changes, since Jackson 2 output is identical.

openliberty/pom.mustache unchanged: it only depends on jackson-annotations, which keeps the com.fasterxml.jackson.core groupId in Jackson 3.

Notes / possible follow-ups:

USE_JACKSON_3 and the Jackson package constants are now duplicated across several generators. Happy to consolidate them into AbstractJavaCodegen in a follow-up PR.
3.1.8 is the latest Jackson 3.1.x; I kept 3.1.2 for consistency with CXF, happy to bump.

Related: #25364 (resteasy client, separate PR).

cc @blaghed

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#25406 [jaxrs-spec] add useJackson3 option