main/postmarketos-ui-os-installer: add support for os-installer 0.5.x

Clayton Craftrequested to merge
craftyguy/os-installer-0.5.x-support into master

There were a number of config changes in 0.5.x (vs 0.4.x), this updates our config to use the latest options/format.

Note: language selection is enabled by default, so no need to list languages.

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