Update dependency grpcio-tools to v1.28.1 (GoogleCloudPlatform#3279)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [grpcio-tools](https://grpc.io) | minor | `==1.27.2` -> `==1.28.1` |

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endpoints/bookstore-grpc-transcoding/requirements.txt

@@ -1,5 +1,5 @@
 grpcio==1.27.2
-grpcio-tools==1.27.2
+grpcio-tools==1.28.1
 google-auth==1.11.2
 six==1.14.0
 googleapis-common-protos==1.51.0

endpoints/bookstore-grpc/requirements.txt

@@ -1,4 +1,4 @@
 grpcio==1.27.2
-grpcio-tools==1.27.2
+grpcio-tools==1.28.1
 google-auth==1.11.2
 six==1.14.0

endpoints/getting-started-grpc/requirements.txt

@@ -1,2 +1,2 @@
 grpcio==1.27.2
-grpcio-tools==1.27.2
+grpcio-tools==1.28.1