Specify handling of timezones in expiration time for onion service second layer plaintext format

https://spec.torproject.org/rend-spec/hsdesc-encrypt.html#second-layer-plaintext says:

    expiration-time: A timestamp in "YYYY-MM-DDTHH:MM:SS" format (iso time
                     with no space) after which the above seed expires and
                     is no longer valid as the input for PoW.

This does not specify how this should be interpreted with regard to timezone. I assume it should be UTC (perhaps it's written elsewhere that all times are UTC? That's what I'd expect), but we should write that down.

Also, we probably shouldn't describe this as "iso time with no space", since it's not. ISO 8601 supports having the timezone encoded in the format or not, dropping and number of fields as long as the more significant fields remain, decimal fractional seconds, etc. I presume we do not support those things.