rel="in-reply-to" is deprecated

[capjamesg]

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The section on federated comments says "When marked up with h-entry and rel=in-reply-to and/or class=u-in-reply-to, your reply can show up as a comment on the original post." The rel="in-reply-to" property is deprecated and beginners may struggle to distinguish the difference between the class and rel implementations.

I was confused when seeing the rel value. It has not been adopted nearly as much as the in-reply-to class, which is documented extensively on the wiki (https://indieweb.org/reply) and in dozens of field examples.

@tantek said on Nov. 9th 2021 "It should be dropped from that completely or maybe given a warning to stop using it" in the IndieWeb Chat, recommending instead to "use u-in-reply-to instead on the actual h-entry reply" (cite: https://chat.indieweb.org/dev/2021-11-09#bottom).

[capjamesg]

@gRegorLove Would you like me to submit a PR that drops documentation for this attribute?