docs: Update README.md - update CI status badge

The previous badges were pointing to old CI (last run in 2021) leading
to false impression that it is tested with Java 7, 8 and 11 only.

I updated to point to new GitHub Ci. I only pointed to the top-level as
I have not found a way to point to specific job of a GitHub workflow.

Author: apupier

README.md

@@ -75,9 +75,8 @@ cd -
 
 Java Version | Status
 ------------ | ------
-Java 7 | [![Kokoro CI](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java7.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java7.html)
-Java 8 | [![Kokoro CI](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java8.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java8.html)
-Java 11 | [![Kokoro CI](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java11.svg)](https://storage.googleapis.com/cloud-devrel-public/java/badges/google-api-java-client/java11.html)
+
+[![GitHub CI](https://github.com/googleapis/google-api-java-client/actions/workflows/ci.yaml/badge.svg)](https://github.com/googleapis/google-api-java-client/actions/workflows/ci.yaml)
 
 ## Dependencies
 This library is built on top of two common libraries, also built by Google, and also designed to