Bump guava from 28.1-jre to 29.0-jre in /invoker/testfunction (GoogleCloudPlatform#86)

Bumps [guava](https://github.com/google/guava) from 28.1-jre to 29.0-jre.
- [Release notes](https://github.com/google/guava/releases)
- [Commits](https://github.com/google/guava/commits)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

invoker/testfunction/pom.xml

@@ -30,7 +30,7 @@
     <dependency>
       <groupId>com.google.guava</groupId>
       <artifactId>guava</artifactId>
-      <version>28.1-jre</version>
+      <version>29.0-jre</version>
     </dependency>
     <dependency>
       <groupId>com.google.code.gson</groupId>