[Tech] Truly ignore flatpak-build

[CommandMC]

This is an alternative PR to #2748
I didn't like the fact that we had to explicitly add the folder when invoking Prettier in the first place. It's already in our .prettierignore, so it should be... ignored. Newer versions of Prettier do this correctly, so this was solved by a simple update (and subsequently fixing all the new formatting rules)
In the process, I noticed that ESLint also had problems with this folder. Adding it to .eslintignore was all it needed

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• N/A - Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

lgtm