routing: stricter maxHTLC checks

We require channel updates to have the max HTLC message flag set.

Several flows need to pass that check before channel updates are
forwarded to peers:
* after channel funding: `addToRouterGraph`
* after receiving channel updates from a peer:
  `ProcessRemoteAnnouncement`
* after we update channel policies: `PropagateChanPolicyUpdate`

docs/release-notes/release-notes-0.16.0.md

@@ -17,6 +17,9 @@ current gossip sync query status.
   bytes](https://github.com/lightningnetwork/lnd/pull/6913). This moves LND
   closer to being spec compliant.
 
+* Channel updates without the maxHTLC message flag set are recognized as invalid
+  and are [not relayed](https://github.com/lightningnetwork/lnd/pull/7415).
+
 ## RPC
 
 * The `RegisterConfirmationsNtfn` call of the `chainnotifier` RPC sub-server