when upgrading > v22.12, uninstall megapixels and install postmarketos-default-camera

To ensure the proper camera application gets installed when upgrading from v22.12 to edge / the upcoming releae, we should replace megapixels with postmarketos-default-camera. On the Librem 5, this will then result in millipixels getting installed, and we can swap the default camera per device later on if needed.

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