keysend plugin: make cltv_expiry for keysends slightly more conservative

This makes the min_cltv_expiry_delta equal to Rust-Lightning's, which is
the highest minimum we know of.

Changelog-Changed: keysend now uses 22 for the final CTLV, making it rust-lightning compatible.

plugins/keysend.c

@@ -163,7 +163,8 @@ static struct command_result *json_keysend(struct command *cmd, const char *buf,
 	p->payment_secret = NULL;
 	p->amount = *msat;
 	p->routes = NULL;
-	p->min_final_cltv_expiry = DEFAULT_FINAL_CLTV_DELTA;
+	// 22 is the Rust-Lightning default and the highest minimum we know of.
+	p->min_final_cltv_expiry = 22;
 	p->features = NULL;
 	p->invstring = NULL;
 	p->why = "Initial attempt";