Include `dotnet_8` image for `egg-t-modloader`

[1zun4]

Description

This will fix the "You must install or upgrade .NET to run this application" issue with newer tModLoader.

I discovered that someone had already reported it (https://github.com/pelican-eggs/eggs/issues/2910), but decided to leave the rest in the dark, but with his "update the yolk" clue, I was able to figure it out myself.

Checklist for all submissions

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

• [x] You verify that the start command applied does not use a shell script
  • [x] If some script is needed then it is part of a current yolk or a PR to add one
• [x] The egg was exported from the panel