deps: update dependency io.grpc:grpc-bom to v1.27.0 (#177)

Author: renovate-bot

google-cloud-bigtable-deps-bom/pom.xml

@@ -80,7 +80,7 @@
     <google.auth.version>0.20.0</google.auth.version>
     <google-http-client.version>1.34.1</google-http-client.version>
     <!-- make sure to update the grpc version in README -->
-    <grpc.version>1.26.0</grpc.version>
+    <grpc.version>1.27.0</grpc.version>
     <guava.version>28.2-android</guava.version>
     <opencensus.version>0.24.0</opencensus.version>
     <protobuf.version>3.11.1</protobuf.version>