[kotlin-server][jaxrs-spec] Use templated value javaxPackage in mustache templates for kotlin-server generator

[gdiegel]

Fixes #15617

This PR switches the hardcoded javax in the import statements with the templated value {{javaxPackage}} in order to have it correctly replaced with the desired package based on the config option useJakartaEe. It includes addes unit tests and test resources.

@jimschubert, @dr4ke616, @karismann, @Zomzog, @andrewemery, @4brunu, @yutaka0m

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[4brunu]

Hi @gdiegel, thanks for creating this PR.
Can you please check if #15593 already fixes this?
Thanks

[gdiegel]

Hi @4brunu, it does, seeing that it contains the same fixes I made and also, importantly, amends the documentation and cli options. This PR #15618 may be closed.

[gdiegel]

Is there going to be a 6.7.0 release before the 7.0.0 release? If yes, when can we expect it to be released?

[wing328]

Is there going to be a 6.7.0 release before the 7.0.0 release? If yes, when can we expect it to be released?

Nope

I hope to release v7.0.0 by the end of Jun