deps: update dependency com.google.guava:guava-bom to v29 (#244)

Author: renovate-bot

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

@@ -81,7 +81,7 @@
  <google-http-client.version>1.34.2</google-http-client.version>
  <!-- make sure to update the grpc version in README -->
  <grpc.version>1.28.1</grpc.version>
- <guava.version>28.2-android</guava.version>
+ <guava.version>29.0-android</guava.version>
  <opencensus.version>0.26.0</opencensus.version>
  <protobuf.version>3.11.4</protobuf.version>
  <threeten.version>1.4.3</threeten.version>