games/minecraft-server: update to 1.20.6

Changes:
https://www.minecraft.net/en-us/article/minecraft-java-edition-1-20-5
https://www.minecraft.net/en-us/article/minecraft-java-edition-1-20-6

This version require at least Java 18 - bump to 21 (next LTS version).
Add possibility to run with different Java versions using environment
variable JAVA_VERSION for standalone server and minecraft_java_version
variable in rc.conf for daemon.
Also several small fixes.

PR: 279204
Tested by: vvd
Approved by: freebsd@jonathanprice.org (maintainer, timeout 20 days)