Upgrade Java 15 version in CI image

[spring-builds]

https://github.com/AdoptOpenJDK/openjdk15-binaries/releases/download/jdk15u-2020-09-16-03-01/OpenJDK15U-jdk_x64_linux_hotspot_2020-09-16-03-01.tar.gz

[snicoll]

Weird that the ga channel for JDK 15 doesn't seem to work yet.