So you can upload the jar and select the java version to use (options are java 8, 11 and 17)
#Ability to use custom server jars
silk dagger
lusty hare
minehut changes the java version depending on your server version
vestal quail
custom server jars were already suggested (https://discord.com/channels/239599059415859200/1006370073281314826)
"likely won't be adding the ability to upload your own version" -Trent