CLN-to-LDK `keysend` is broken due to incompatible `min_final_cltv_expiry`

[tnull]

In keysend.c, there currently is a remark:

	// 22 is the Rust-Lightning default and the highest minimum we know of.
	p->min_final_cltv_expiry = 22;

While this historically might have been accurate, it isn't anymore (at the very least since Feb. 2025, cf. lightningdevkit/rust-lightning#3608).

Our current value for MIN_FINAL_CLTV_EXPIRY_DELTA is 42, and correspondingly CLN-to-LDK keysend won't work currently.

Maybe the value in keysend.c should be bumped and/or make it a configurable parameter that the user can override?