Bump version to v0.21.1

Author: albertomercurio

Project.toml

@@ -1,7 +1,7 @@
 name = "QuantumToolbox"
 uuid = "6c2fb7c5-b903-41d2-bc5e-5a7c320b9fab"
 authors = ["Alberto Mercurio", "Yi-Te Huang"]
-version = "0.21.0"
+version = "0.21.1"
 
 [deps]
 ArrayInterface = "4fba245c-0d91-5ea0-9b3e-6abc04ee57a9"