Allow cirq-google installation with protobuf-5 (#7147)

Unit tests pass with protobuf-5.29.3 (latest in the 5.x series).
Python protobuf sources are still generated with protobuf-4
so cirq-google can be used in environments with protobuf-4 too.

Related to #7145

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: pavoljuhas

cirq-google/requirements.txt

@@ -1,4 +1,4 @@
 google-api-core[grpc] >= 1.14.0
 proto-plus >= 1.20.0
-protobuf ~= 4.25
+protobuf >= 4.25, <6.0
 typedunits