Fix FTB Egg

[MadsKelberg]

Description

The FTB Egg does not currently work with the new FTB API #53.

Added Java 21 docker image, which is required by some new modpacks.

The install script used jdk 8 slim, which caused trouble when downloading the modpack server from FTB.
Instead it now uses jdk 21 slim.

The symlink for NeoForge unix_args was looking at the wrong directory, so that was also fixed.

I tested it with the FTB Direwolf20 newest version (1.8.2) and FTB Skies: Expert oldest version (1.0.1)

Checklist for all submissions

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you tested and reviewed your changes with confidence that everything works?
• Did you branch your changes and PR from that branch and not from your master branch?

New egg Submissions

1. Does your submission pass tests (server is connectable)?
2. Does your egg use a custom docker image?
   • Have you tried to use a generic image?
   • Did you PR the necessary changes to make it work?
3. Have you added the egg to the main README.md and any other README files in subdirectories of the egg (e.g /game_eggs) according to the alphabetical order?
4. Have you added a unique README.md for the egg you are adding according to the alphabetical order?
5. You verify that the start command applied does not use a shell script
   • If some script is needed then it is part of a current yolk or a PR to add one
6. The egg was exported from the panel

[MadsKelberg]

This should also resolve #46

[QuintenQVD0]

This fille should use the egg-pterodactyl-name.json as it is exported from pterodactyl, all features are missing from the egg and please don't use the pterodactyl images as they have not been updated for months

[MadsKelberg]

@QuintenQVD0 Okay, I will update the name.
What do you mean by all features are missing from the egg?
Which images am I suppose to use if not pterodactyl images?

[QuintenQVD0]

@QuintenQVD0 Okay, I will update the name. What do you mean by all features are missing from the egg? Which images am I suppose to use if not pterodactyl images?

"features": null, -->

        "features": [
        "eula",
        "java_version",
        "pid_limit"
    ],

https://github.com/pelican-eggs/yolks?tab=readme-ov-file#java