Update gunicorn requirement from <21,>=20 to >=20,<22 (#1045)

Author: dependabot[bot]

requirements/adapter.txt

@@ -18,5 +18,5 @@ sanic>=22,<23; python_version>"3.6"
 starlette>=0.14,<1
 tornado>=6,<7
 uvicorn<1                                # The oldest version can vary among Python runtime versions
-gunicorn>=20,<21
+gunicorn>=20,<22
 websocket_client>=1.2.3,<2               # Socket Mode 3rd party implementation