sauron: estimatefees has changed in CLN version v23.05

[tonyaldon]

estimatefees has changed in CLN version v23.05.

See deprecated section in https://github.com/ElementsProject/lightning/blob/master/CHANGELOG.md#23051---2023-06-05-austin-texas-agreementatxa-ii.

The new spec is (https://github.com/ElementsProject/lightning/blob/master/doc/PLUGINS.md#estimatefees):

estimatefees

Polled by lightningd to get the current feerate, all values must be passed in sat/kVB.

The plugin must return feerate_floor (e.g. 1000 if mempool is
empty), and an array of 0 or more feerates. Each element of
feerates is an object with blocks and feerate, in
ascending-blocks order, for example:

{
	"feerate_floor": <sat per kVB>,
	"feerates": {
		{ "blocks": 2, "feerate": <sat per kVB> },
		{ "blocks": 6, "feerate": <sat per kVB> },
		{ "blocks": 12, "feerate": <sat per kVB> }
		{ "blocks": 100, "feerate": <sat per kVB> }
	}
}

lightningd will currently linearly interpolate to estimate between given blocks (it will not extrapolate, but use the min/max blocks values).