This was changed as part of b049c057 (upgrade to slapos.toolbox version
0.123, 2021-07-19), but this is not a slapos.toolbox promise plugin, it's
still an old style promise.

This introduces some breaking changes to the check_url_available
promise:

- http_code has been renamed to http-code
- check-secure = 1 has been deprecated; http-code = 401 should be used
  instead

https://github.com/restic/rest-server