deps: update dependency io.grpc:grpc-bom to v1.28.0 (#178)

Author: renovate-bot

pom.xml

@@ -160,7 +160,7 @@
     <google.auth.version>0.20.0</google.auth.version>
     <google.api.version>1.30.9</google.api.version>
     <google.http.version>1.34.2</google.http.version>
-    <grpc.version>1.27.2</grpc.version>
+    <grpc.version>1.28.0</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <opencensus.version>0.24.0</opencensus.version>
     <annotations-api.version>1.3.2</annotations-api.version>