Adopt snake case as a standard for Meta v4

[EnnuiL]

Currently, Meta v3 adopts both kebab-case (ex: the v3/versions/quilt-mappings route) and camelCase (ex: the gameVersion field on all mappings objects); Considering that the project in general is pushing for snake case as a standard case, the meta should also follow these steps and adopt it too; Applying this change should be simple, considering that the above examples of casing are likely to be the only ones on the meta itself
The exceptions for this is of course, the launcher profile metadata, which isn't really meta-y but rather "hey installers! we're giving you a file for free", with those files being meant to be read by Minecraft launchers in the end, who expect camelCase

For an example of it being used for API, see Modrinth's meta