subd: Do not send feerate updates to non-channeld subds

[cdecker]

Turns out we were sending feerate updates to daemons that do not understand it. Don't do that!

Closes #6932

Changelog-Fixed: channeld: We could crash closingd by sending it a channeld message

[whitslack]

@cdecker: I ran into this today too. Wouldn't a cleaner fix be to drop CLOSINGD_SIGEXCHANGE from the set of channel_states for which channel_state_fees_can_change returns true?

https://github.com/ElementsProject/lightning/blob/v23.11/lightningd/channel.h#L518

Then the !channel_state_fees_can_change(channel->state) check at the top of try_update_feerates(…) would fail early.

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Addendum: Note that in v23.05.2, try_update_feerates would test channel_fees_can_change, which would return true only if the channel state was CHANNELD_NORMAL or CHANNELD_SHUTTING_DOWN. But then 0b4622b replaced channel_fees_can_change by channel_state_fees_can_change and inexplicably added CLOSINGD_SIGEXCHANGE as a state in which fees can change.