doc: mark callback-based fs API as legacy
A few folks have voiced support for bringing new features only to the Promise-based and sync APIs of fs
in https://github.com/nodejs/node/pull/37944. This PR marks the callback-API as "Legacy" to reflect this position.
@nodejs/fs wdyt?
For reference, "Legacy" status is defined by https://github.com/nodejs/node/blob/ed6e8f6518e98b26fe58d29669de734d3120708a/doc/api/documentation.md#L42-L44