Decomission Java 9 in CI pipeline

[keilw]

Based on the error log:

Downloading JDK from https://download.java.net/java/early_access/jdk19/12/GPL/openjdk-19-ea+12_linux-x64_bin.tar.gz
https://download.java.net/java/GA/jdk9/9.0.4/binaries/openjdk-9.0.4_linux-x64_bin.tar.gz...
The command "~/bin/install-jdk.sh --target "/home/travis/openjdk9" --workspace "/home/travis/.cache/install-jdk" --feature "9" --license "GPL" --cacerts" failed and exited with 8 during .

In the current Travis-CI build it looks like Java 9 no longer works there, so as Java 11 works and it's an LTS we should decomission Java 9.