gossipd: don't prune if we haven't seen on side's update at all.

This caused a "flake" in testing, because it's wrong:

```
_____________________________ test_gossip_pruning ______________________________
[gw2] linux -- Python 3.10.16 /home/runner/.cache/pypoetry/virtualenvs/cln-meta-project-AqJ9wMix-py3.10/bin/python

node_factory = <pyln.testing.utils.NodeFactory object at 0x7f0267530490>
bitcoind = <pyln.testing.utils.BitcoinD object at 0x7f0267532b30>

    def test_gossip_pruning(node_factory, bitcoind):
        """ Create channel and see it being updated in time before pruning
        """
        l1, l2, l3 = node_factory.get_nodes(3, opts={'dev-fast-gossip-prune': None,
                                                     'allow_bad_gossip': True,
                                                     'autoconnect-seeker-peers': 0})
    
        l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
        l2.rpc.connect(l3.info['id'], 'localhost', l3.port)
    
        scid1, _ = l1.fundchannel(l2, 10**6)
        scid2, _ = l2.fundchannel(l3, 10**6)
    
        mine_funding_to_announce(bitcoind, [l1, l2, l3])
        wait_for(lambda: l1.rpc.listchannels(source=l1.info['id'])['channels'] != [])
        l1_initial_cupdate_timestamp = only_one(l1.rpc.listchannels(source=l1.info['id'])['channels'])['last_update']
    
        # Get timestamps of initial updates, so we can ensure they change.
        # Channels should be activated locally
>       wait_for(lambda: [c['active'] for c in l1.rpc.listchannels()['channels']] == [True] * 4)
```

Here you can see it has pruned:

```
lightningd-1 2025-01-24T07:39:40.873Z DEBUG   gossipd: Pruning channel 105x1x0 from network view (ages 1737704380 and 0)
...
lightningd-1 2025-01-24T07:39:50.941Z UNUSUAL lightningd: Bad gossip order: could not find channel 105x1x0 for peer's channel update
```

Changelog-Fixed: Protocol: we were overzealous in pruning channels if we hadn't seen one side's gossip update yet.
Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

gossipd/gossmap_manage.c

@@ -320,15 +320,15 @@ static void remove_channel(struct gossmap_manage *gm,
 	}
 }
 
+/* If we don't know, we assume it's good */
 static u32 get_timestamp(struct gossmap *gossmap,
 			 const struct gossmap_chan *chan,
 			 int dir)
 {
 	u32 timestamp;
 
-	/* 0 is sufficient for our needs */
 	if (!gossmap_chan_set(chan, dir))
-		return 0;
+		return UINT32_MAX;
 
 	gossmap_chan_get_update_details(gossmap, chan, dir,
 					&timestamp,