chore: pin minimum supported protobuf version for testing (#273)

See:
https://github.com/googleapis/proto-plus-python/pull/270/files#r737644737

Author: tseaver

packages/proto-plus/testing/constraints-3.6.txt

@@ -6,3 +6,4 @@
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
 google-api-core==1.22.2
+protobuf==3.19.0