feat(api): also clear cache when leaving a guild

Catalogger.Backend/Cache/InMemoryCache/ChannelCache.cs

@@ -81,4 +81,17 @@ public class ChannelCache
                 .Select(id => _channels.GetValueOrDefault(id))
                 .Where(c => c != null)
                 .Select(c => c!);
+
+    public void RemoveGuild(Snowflake guildId)
+    {
+        if (!_guildChannels.TryGetValue(guildId, out var channelIds))
+            return;
+
+        foreach (var id in channelIds)
+        {
+            _channels.Remove(id, out _);
+        }
+
+        _guildChannels.Remove(guildId, out _);
+    }
 }