Remove RSA-SHA256-TLSSecrets link authentication

Nick Mathewsonrequested to merge
nickm/tor:too_many_tlssecrets into main

This link authentication mechanism is deprecated because it uses 1024-bit RSA keys, and because it requires access to the TLS "master secret", which most reasonable TLS implementations don't expose.

The current mechanism (Ed25519-SHA256-RFC5705) was introduced in 0.3.0.1-alpha, and is now ubiquitous.

(This branch is based on !861 (merged) ; please review that one first.)

Closes #41020 (closed).

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