Enable DISABLE_DISABLING_ED25519

Split from #18319 (moved)

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 (moved).)

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.