meson: Require libssh-0.8.1 or newer

meson: Require libssh-0.8.1 or newer

According to repology.org:

              RHEL-8: 0.9.4
              RHEL-9: 0.9.6
           Debian 11: 0.9.5
  openSUSE Leap 15.3: 0.8.7
        Ubuntu 20.04: 0.9.3

And the rest of distros has something newer anyways. Requiring
0.8.1 or newer allows us to drop the terrible hack where we
rename functions at meson level using #define. Note, 0.8.0 is
the version of libssh where the rename happened. It also allows
us to stick with SHA-256 hash algorithm for public keys.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>