Remove jackson requirement, fixes #6881

[rignicat]

Jackson was a requirement when using the server JAX-RS generator. Removing this requirement allows generating multi-platform code.

The PR is split into 3 commits (with a 4th for updating the samples/docs):

1. Allow enabling/disabling jackson, since it was always enabled
2. Disabled generating RootResource and RootApplication if generating interfaces, since these files make no sense in the context of generating interfaces
3. Update templates to remove jackson references when disabled

The 2nd can be its own PR if required, since not directly relevant to the linked issue. But its a small change and something that makes no sense as it was, so I just added it instead of having to get a reviewer's attention twice.

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[wing328]

please also review the build failure when you've time