Update fixedbitset requirement from 0.3 to 0.4 (#1726)

Updates the requirements on [fixedbitset](https://github.com/bluss/fixedbitset) to permit the latest version.
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crates/bevy_ecs/Cargo.toml

@@ -25,7 +25,7 @@ bevy_ecs_macros = { path = "macros", version = "0.4.0" }
 
 async-channel = "1.4"
 bitflags = "1.2"
-fixedbitset = "0.3"
+fixedbitset = "0.4"
 fxhash = "0.2"
 thiserror = "1.0"
 downcast-rs = "1.2"