games/minecraft-server: Fix java version in rc script

The service fails to start - the default value for the Java version was
"21+" instead of the specific "N" selected during build according to
DEFAULT_VERSIONS+=java=N.
Replace JAVA_VERSION with JAVA_PORT_VERSION in SUB_LIST.

Approved by: blanket (fix runtime)
MFH: 2026Q1

(cherry picked from commit 7847d2d4477afd99b738a5bc6a8f07c05d14cb81)