Switch lock backport package to #if defs instead (#3217)

src/Discord.Net.Rest/Net/Queue/RequestQueueBucket.cs

@@ -17,7 +17,11 @@ namespace Discord.Net.Queue
     {
         private const int MinimumSleepTimeMs = 750;
 
+#if NET9_0_OR_GREATER
         private readonly Lock _lock;
+#else
+        private readonly object _lock;
+#endif
         private readonly RequestQueue _queue;
         private int _semaphore;
         private DateTimeOffset? _resetTick;