docs(autodoc) add upgrade docs for 2.4.0 (#7023)
docs(autodoc) add upgrade docs for 2.4.0 (#7023)

* docs(autodoc) introduce autodoc for upgrading.md

* docs(upgrade) syncrhonize with docs' upgrading.md

The `upgrading.md` file in
https://docs.konghq.com/gateway-oss/2.3.x/upgrading/ was produced by
manually copying the first part of the UPGRADE.md file on this repo and
then doing manual edits.

This applies several of those manual updates to UPGRADE.md
so that future docs versions can be automatically generated by a
script.

* docs(UPGRADE) add upgrade docs for 2.4.0

* docs(upgrade) add 2.4.0 upgrade path

* release: 2.4.0

* docs(pdk) document underscore transformations in kong.response.set_headers (#7018)

Related with #6995

Co-authored-by: Vinicius Mignot <16989b0990827e84dc3edf87410a8169e8edb2d6@gmail.com>
Kong/kong,Kong/kong,Kong/kong