bump version from `0.21.0rc3` to `0.21.0rc4`

pyproject.toml

 [tool.poetry]
 name = "supervision"
-version = "0.21.0rc3"
+version = "0.21.0rc4"
 description = "A set of easy-to-use utils that will come in handy in any Computer Vision project"
 authors = ["Piotr Skalski <piotr.skalski92@gmail.com>"]
 maintainers = ["Piotr Skalski <piotr.skalski92@gmail.com>"]