java template does not set JAVA_HOME correct

The template uses ms openjdk and adds support for mvn and gradle via the java feature.

```
{
"name": "Java",
// Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
"image": "mcr.microsoft.com/devcontainers/java:1-21-bullseye",

"features": {
"ghcr.io/devcontainers/features/java:1": {
"version": "none",
"installMaven": "true",
"installGradle": "true"
}
}
}
```

The container than has JAVA_HOME pointing to `/usr/local/sdkman/candidates/java/current` which does not exist.
MS jdk is installed at `/usr/lib/jvm/msopenjdk-current/bin/java`

```
$ mvn
The JAVA_HOME environment variable is not defined correctly,
this environment variable is needed to run this program.
```