Update README to remove version specificity for warning

Author: ScarletKuro

README.md

@@ -131,7 +131,7 @@ var deserializedOrder = JsonSerializer.Deserialize<Order>(json);
 
 **❌ Cons:**
 - Only works with reflection-based serialization
-- Produces SYSLIB1223 warning with source generators (.NET 6-8)
+- Produces SYSLIB1223 warning with source generators
 - No AOT (Ahead-of-Time) compilation support
 
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