deps: update dependency io.grpc:grpc-bom to v1.28.0 (#71)

Author: renovate-bot

pom.xml

@@ -67,7 +67,7 @@
     <google.api-common.version>1.8.1</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
     <gax.version>1.54.0</gax.version>
-    <grpc.version>1.27.2</grpc.version>
+    <grpc.version>1.28.0</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <junit.version>4.13</junit.version>
     <guava.version>28.2-android</guava.version>