F-Droid 2: Change how we pre-install the microG F-Droid repo

We currently use the files in [additional_repos.xml](https://github.com/lineageos4microg/android_vendor_partner_gms/tree/master/additional_repos.xml) According to [this post in the f-Droid blog / news](https://f-droid.org/en/2026/03/28/how-to-include-repos-in-rom.html) we need to use a JSON file instead of the XML file we currently use. The new JSON file can live in the same place as the existing XML file: `/product/etc/org.fdroid.fdroid/` This change needs to be made at the same time (or before) we ship F-Droid 2.0. As the blog says > The old method of adding repositories using an XML file will not work in F-Droid 2.0. We suggest to ship both, the JSON and the XML file as soon as possible until F-Droid 2.0 has stabilized.