build: bump gunicorn to latest version, just published

We previously locked to a specific gunicorn commit because of an
incompatibility with recent eventlet versions. gunicorn now released a
new version that includes this patch, so we can use it instead.

requirements/requirements.prod.txt

@@ -1 +1 @@
-gunicorn @ git+https://github.com/benoitc/gunicorn@add8a4c951f02a67ca1f81264e5c107fa68e6496
+gunicorn>=21.1.0