deps: update dependency com.google.auth:google-auth-library-bom to v0.20.0 (#168)

Author: renovate-bot

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

@@ -77,7 +77,7 @@
     <google.api-common.version>1.8.1</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
     <google.core.version>1.92.2</google.core.version>
-    <google.auth.version>0.19.0</google.auth.version>
+    <google.auth.version>0.20.0</google.auth.version>
     <google-http-client.version>1.34.0</google-http-client.version>
     <!-- make sure to update the grpc version in README -->
     <grpc.version>1.26.0</grpc.version>