deskutils/freeplane: Set JAVA_VERSION

Freeplane fails to start with the current JAVA_DEFAULT (25) with
the error message:

ERROR: Found /usr/local/bin/java in $PATH.
ERROR: It has version 25.0.2
ERROR: Currently, freeplane requires java version 8 or from 11 to 23
ERROR:
ERROR: Select a supported java version
ERROR: by setting FREEPLANE_JAVA_HOME to a valid java location
ERROR: OR use an unsupported java version
ERROR: by setting FREEPLANE_USE_UNSUPPORTED_JAVA_VERSION to 1

So set JAVA_VERSION to a list of supported JDK.

Note that if you start freeplane with the environment variable
FREEPLANE_USE_UNSUPPORTED_JAVA_VERSION=1, then the program fails with
the error message:

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