Remove `channel_update` from Bolt 4 error messages

[t-bast]

While implementing a prototype for taproot gossip (#1059), one pain point is that Bolt 4 failure messages sometimes embed a channel_update message (which will be a "legacy" message once we introduce channel_update_2). We've discussed this several times in the past, and if I remember correctly, we were in favor of changing these failure messages (or introducing new ones) to contain the channel update fields we care about (fees and cltv_expiry_delta, maybe others) instead of embedding an entire channel_update message.

The upside is that this will let us deprecate channel_update at some point, but the downside is that if we add new fields to channel_update(_2), those won't be automatically included in Bolt 4 failure messages.

I think we should change the Bolt 4 failure messages to include a TLV stream that contains channel update fields. This is something we can introduce right now, which will make our lives a bit simpler when shipping taproot gossip and gossip v1.5 / 1.75 / 2.0 / N.M ™️

If I open a PR to do that, are implementations willing to implement this?

[t-bast]

As discussed during the spec meeting, this should be part of #1059.