GET /config and GET /config/{subject} incorrect return format

[granders]

According to documentation, the response to GET on /config and /config/{subject} should be json with

{ "compatibility": }

Instead, it is

{ "compatibilityLevel": }

This is inconsistent with the return type from POST requests to the same endpoints, and inconsistent with documentation. The returned JSON should have the field "compatibility"

[ewencp]

We need to be careful about compatibility here (no pun intended). We should leave the compatibilityLevel field in, fix it to include compatibility, and updated the docs to clarify that compatibilityLevel is deprecated.

[mageshn]

@joel-hamill https://docs.confluent.io/current/schema-registry/docs/using.html#getting-the-top-level-config should be fixed to say compatibility instead of compatibilityLevel

[th0ger]

Hey congrats, this issue just turned 10 y.o. Doesn't sound that complicated?