fix(deps): Add protobuf version lower bound

This requirement was forced by #120 and ideally should have been documented as part of the 4.0.0 release.

PiperOrigin-RevId: 477344323

Author: cloud-profiler-team

setup.py

@@ -30,7 +30,7 @@
     'google-api-python-client!=1.12.0,!=2.0.2',
     'google-auth>=1.0.0',
     'google-auth-httplib2',
-    'protobuf',
+    'protobuf>=3.20',
     'requests',
 ]