[Bug 2130273] [NEW] do-release-upgrade deletes sources.list format entries during update to 25.10

[Launchpad Bug Tracker]

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To reproduce:
1. On Ubuntu 25.04, add a repository in the old sources.list format (https://wiki.debian.org/SourcesList#sources.list_format). For example, I've added the Mozilla Firefox repo via following https://support.mozilla.org/en-US/kb/install-firefox-linux "For Debian Bookworm or older" instructions.
2. Start upgrade process to 25.10 via do-release-upgrade. In the beginning of the process, the sources.list format repos will be moved into e.g. "mozilla.list.migrate", while the deb822 repos will be copied into e.g. "ubuntu.sources.distUpgrade" 
3. Finish upgrade. All files with deb822 format will still be there, but mozilla.list will be gone entirely.

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New

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do-release-upgrade deletes sources.list format entries during update to 25.10
https://bugs.launchpad.net/bugs/2130273
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