CI, use older protobuf for older TF, follow-up fix

Follow-up fix for #1209

.github/workflows/main.yml

@@ -287,7 +287,7 @@ jobs:
             pip install --user numpy==1.19.5
             # Older TF needs also older protobuf version.
             # https://github.com/rwth-i6/returnn/issues/1209
-            pip install --user protobuf==3.20.1
+            pip install --user "protobuf<=3.20.1"
           fi
         fi
         pip install --user -r requirements.txt | cat