get_channel_archived_public_threads() seems to not be using ISO8601 for the before parameter

[Sirraide]

get_channel_archived_public_threads() (and possibly get_channel_archived_private_threads(); I’m not sure if that one is also affected) optionally takes a before parameter that is currently just an Option<u64> that gets converted to a string and then sent to the route, from what I can tell. However, the Discord API docs specify that this parameter should be in the ISO8601 format.

As a result, when trying to pass e.g. a unix timestamp as a u64 to get_channel_archived_public_threads(), I get the following error message after trying to execute this against a forum channel:

Invalid Form Body (before: Could not parse 1664162557. Should be ISO8601.)

[jamesbt365]

This is a duplicate of #2664 and is fixed in #2665.