pants: upgrade Pants to 2.21.0 (#17)

Author: AlexTereshenkov

pants.toml

@@ -2,7 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 [GLOBAL]
-pants_version = "2.20.0"
+pants_version = "2.21.0"
 backend_packages = ["pants.backend.experimental.go"]
 
 [anonymous-telemetry]