deps: update opencensus.version to v0.26.0 (#223)

Author: renovate-bot

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

@@ -82,7 +82,7 @@
     <!-- make sure to update the grpc version in README -->
     <grpc.version>1.28.0</grpc.version>
     <guava.version>28.2-android</guava.version>
-    <opencensus.version>0.25.0</opencensus.version>
+    <opencensus.version>0.26.0</opencensus.version>
     <protobuf.version>3.11.4</protobuf.version>
     <threeten.version>1.4.2</threeten.version>
   </properties>