refactor: don't use 'out' in IDiscordCache methods

this change is required for async cache (.NET doesn't support async ref/out params)

PluralKit.Bot/Commands/ServerConfig.cs

@@ -110,7 +110,7 @@ namespace PluralKit.Bot
 
             // Resolve all channels from the cache and order by position
             var channels = (await Task.WhenAll(blacklist.Blacklist
-                .Select(id => _cache.GetChannelOrNull(id))))
+                .Select(id => _cache.TryGetChannel(id))))
                 .Where(c => c != null)
                 .OrderBy(c => c.Position)
                 .ToList();

PluralKit.Bot/Commands/SystemEdit.cs

@@ -436,7 +436,7 @@ namespace PluralKit.Bot
         {
             ctx.CheckSystem();
 
-            var guild = ctx.MatchGuild() ?? ctx.Guild ??
+            var guild = await ctx.MatchGuild() ?? ctx.Guild ??
                 throw new PKError("You must run this command in a server or pass a server ID.");
 
             var gs = await _repo.GetSystemGuild(guild.Id, ctx.System.Id);