Replace Phantom Forces with Frontlines in the "Design for Roblox" page

[DotLYHiyou]

Changes

Phantom forces had multiple allegations regarding the safety of their users among other things. You can view said allegations summed up in 60 seconds here:

• https://www.youtube.com/shorts/lx1H9gQWn7A

I felt like HELLMET was a better replacement for Phantom Forces as it accurately depicts what older audiences would want to play.

Checks

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[github-actions]

Hi @DotLYHiyou, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.