SWAPDB: don't test against 3.x

src/StackExchange.Redis/RedisFeatures.cs

@@ -140,6 +140,11 @@ public RedisFeatures(Version version)
         /// </summary>
         public bool StringSetRange => Version >= v2_1_8;
 
+        /// <summary>
+        /// Is SWAPDB available?
+        /// </summary>
+        public bool SwapDB => Version >= v4_0_0;
+
         /// <summary>
         /// Does TIME exist?
         /// </summary>

tests/StackExchange.Redis.Tests/Databases.cs

@@ -88,6 +88,8 @@ public async Task SwapDatabases()
         {
             using (var muxer = Create(allowAdmin: true))
             {
+                Skip.IfMissingFeature(muxer, nameof(RedisFeatures.SwapDB), r => r.SwapDB);
+
                 RedisKey key = Me();
                 var db0id = TestConfig.GetDedicatedDB(muxer);
                 var db0 = muxer.GetDatabase(db0id);
@@ -122,6 +124,8 @@ public async Task SwapDatabasesAsync()
         {
             using (var muxer = Create(allowAdmin: true))
             {
+                Skip.IfMissingFeature(muxer, nameof(RedisFeatures.SwapDB), r => r.SwapDB);
+
                 RedisKey key = Me();
                 var db0id = TestConfig.GetDedicatedDB(muxer);
                 var db0 = muxer.GetDatabase(db0id);