Deprecate GraphQL `feature_flag` argument

What does this MR do and why?

This is a first iteration of #369202 (closed) which documents that the feature_flag property is deprecated and should no longer be used. The property is renamed to _deprecated_feature_flag so developers are aware even if they have not read the docs.

The developer docs are updated. The update also includes new advice for using flags with schema items besides fields, and where the flag is used to toggle behavior.

There should be no functional changes.

Future iterations will replace each use of _deprecated_feature_flag with a feature flag toggle, and the alpha property if the flag is disabled by default.

MR acceptance checklist

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Related to #369202 (closed)