update protobuf constraint

Author: TrevorBergeron

packages/bigframes/testing/constraints-3.10.txt

@@ -98,7 +98,7 @@ pluggy==1.6.0
 prompt_toolkit==3.0.52
 propcache==0.4.1
 proto-plus==1.27.1
-protobuf==4.25.8
+protobuf==5.26.0
 psygnal==0.15.1
 ptyprocess==0.7.0
 pure_eval==0.2.3