chore: update minimum java version to java 8 (#1281)

Source-Link: googleapis/synthtool@b9ad8c4
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:a744afda1956beb7bd6dc8d0b740a7839bbe402e34e2f4ef24589074db53ceb0

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/cloud-devrel-public-resources/owlbot-java:latest
-  digest: sha256:14ecf64ec36f67c7bf04e3dc0f68eafcc01df3955121c38862b695e2ae7515d8
+  digest: sha256:a744afda1956beb7bd6dc8d0b740a7839bbe402e34e2f4ef24589074db53ceb0

README.md

@@ -251,7 +251,7 @@ To get help, follow the instructions in the [shared Troubleshooting document][tr
 
 ## Supported Java Versions
 
-Java 7 or above is required for using this client.
+Java 8 or above is required for using this client.
 
 Google's Java client libraries,
 [Google Cloud Client Libraries][cloudlibs]
@@ -320,7 +320,6 @@ Apache 2.0 - See [LICENSE][license] for more information.
 
 Java Version | Status
 ------------ | ------
-Java 7 | [![Kokoro CI][kokoro-badge-image-1]][kokoro-badge-link-1]
 Java 8 | [![Kokoro CI][kokoro-badge-image-2]][kokoro-badge-link-2]
 Java 8 OSX | [![Kokoro CI][kokoro-badge-image-3]][kokoro-badge-link-3]
 Java 8 Windows | [![Kokoro CI][kokoro-badge-image-4]][kokoro-badge-link-4]