Migrate sshd to clanServices

- migrate service
- add deprecation notice to old module
- add check
- update docs

checks/flake-module.nix

@@ -53,6 +53,7 @@ in
               dummy-inventory-test = import ./dummy-inventory-test nixosTestArgs;
               borgbackup = import ./borgbackup nixosTestArgs;
               data-mesher = import ./data-mesher nixosTestArgs;
+              syncthing = import ./syncthing nixosTestArgs;
             }
             // lib.optionalAttrs (pkgs.stdenv.hostPlatform.system == "aarch64-linux") {
               # for some reason this hangs in an odd place in CI, but it works on my machine ...