KEP-2338: Update to being in line with modern KEP practices

[adrianmoisey]

• One-line PR description: KEP-2338 has been in beta (default to on) since 1.16. This PR is an attempt to modernise the KEP so that the KEP can be graduated to stable, and feature gate removed.

• Issue link: API gzip compression support #2338

• Other comments: Given that this feature has been default to on since 1.16, I believe that it is already effectively GA. This PR is an attempt to follow process and get the feature promoted to stable so that one day the gate can be cleaned up. It's a bit tough digging in the archives and updating a KEP that has already been done, so I'm looking for some help and guidance here.

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