[tor-bugs] #20522 [Core Tor/Tor]: Enable DISABLE_DISABLING_ED25519

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#20522: Enable DISABLE_DISABLING_ED25519
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     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.0.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-ed25519-proto
Actual Points:                |  Parent ID:
       Points:  0.5           |   Reviewer:
      Sponsor:                |
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 Split from #18319

 At some point, we should require relays that once had an ed25519 key
 associated with their RSA key to always have that key, rather than
 allowing them to drop back to a version that didn't support ed25519.

 (This means they need to use a new RSA key to downgrade to an older
 version of tor without ed25519, which is consistent with the pinning in
 #18319.)

 This means either:
 1a. waiting until 0.2.5 is no longer recommended, or
 1b. look at historical metrics data to see how often relays run a recent
 version for a while, then drop back to an older one. If the answer is
 "almost never" then we can just turn it on now.

 To implement this change, replace `#undef DISABLE_DISABLING_ED25519` with
 `#define DISABLE_DISABLING_ED25519`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20522>
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