Chore: Clean up kong.conf.default (#7490)

* testing

* update config doc

* remove immunity_enabled

* readd prefix

* fix lena suggestions

* more suggestions

* more changes

* more lena fixes

* more fixes

* Update app/_src/gateway/reference/configuration/configuration-3.7.x.md

Co-authored-by: lena-larionova <54370747+lena-larionova@users.noreply.github.com>

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Co-authored-by: lena-larionova <54370747+lena-larionova@users.noreply.github.com>