Set release versions to None for dev branch.

gym-unity/gym_unity/__init__.py

 __version__ = "0.20.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_6"
+__release_tag__ = None

ml-agents-envs/mlagents_envs/__init__.py

 __version__ = "0.20.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_6"
+__release_tag__ = None

ml-agents/mlagents/trainers/__init__.py

 __version__ = "0.20.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_6"
+__release_tag__ = None

utils/validate_release_links.py

 ALLOW_LIST = {
     # Previous release table
     "README.md": re.compile(r"\*\*Release [0-9]+\*\*"),
-    "com.unity.ml-agents/Runtime/Actuators/IActionReceiver.cs": RELEASE_PATTERN,
-    "com.unity.ml-agents/Runtime/Actuators/IDiscreteActionMask.cs": RELEASE_PATTERN,
     "docs/Versioning.md": None,
     "com.unity.ml-agents/CHANGELOG.md": None,
     "utils/make_readme_table.py": None,