libssh: add support for SHA256 host public keys

Reported-by: Joshua Rogers
Fixes #21605

Closes #21607

docs/cmdline-opts/hostpubsha256.md

@@ -18,6 +18,3 @@ Example:
 
 Pass a string containing a Base64-encoded SHA256 hash of the remote host's
 public key. curl refuses the connection with the host unless the hashes match.
-
-This feature requires libcurl to be built with libssh2 and does not work with
-other SSH backends.

docs/libcurl/opts/CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256.md

@@ -73,10 +73,6 @@ int main(void)
 }
 ~~~
 
-# NOTES
-
-Requires the libssh2 backend.
-
 # %AVAILABILITY%
 
 # RETURN VALUE