Hi, I updated my MineOS to Java 11. Don't know if the method I used is any good but I'm posting the steps just in case it helps anyone. I did it was as so:
In the MineOS Docker console, I used:
pacman -sS java | grep jre
This shows a list of available Java version for install.
pacman -S jre11-openjdk-headless
This install the version of Java/Openjdk you want, in this case 11. It will ask confirmation, just type Y and press return.
archlinux-java status
Shows the versions of Java installed, the new one should appear in the list, Java 8 will still be the default.
archlinux-java set java-11-openjdk
This sets the new version, in this case Java-11, as the default java version.
Now, just reboot your server (no need to reboot the container) and it should be running Java 11.
I personally kept Java 8 installed just in case I encounter any problems but no longer have the Paper upgrade message, have been running 11 for only one day but have not yet encountered any problems.
Sources:
https://wiki.archlinux.org/index.php/java
https://www.tecmint.com/install-java-on-arch-linux/