htlcswitch+go.mod: use updated fn.ContextGuard

This commit updates the fn dep to the version containing the updates to
the ContextGuard implementation. Only the htlcswitch/link uses the guard
at the moment so this is updated to make use of the new implementation.

go.mod

@@ -36,7 +36,7 @@ require (
 	github.com/lightningnetwork/lightning-onion v1.2.1-0.20240712235311-98bd56499dfb
 	github.com/lightningnetwork/lnd/cert v1.2.2
 	github.com/lightningnetwork/lnd/clock v1.1.1
-	github.com/lightningnetwork/lnd/fn/v2 v2.0.4
+	github.com/lightningnetwork/lnd/fn/v2 v2.0.8
 	github.com/lightningnetwork/lnd/healthcheck v1.2.6
 	github.com/lightningnetwork/lnd/kvdb v1.4.12
 	github.com/lightningnetwork/lnd/queue v1.1.1