[Merge] ~nteodosio/update-manager:main into update-manager:main

[Nathan Teodosio]

Nathan Teodosio has proposed merging ~nteodosio/update-manager:main into update-manager:main.

Requested reviews:
  Sebastien Bacher (seb128)
Related bugs:
  Bug #2043425 in update-manager (Ubuntu): "Details pane against unavailable Pro updates is misleading"
  https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/2043425

For more details, see:
https://code.launchpad.net/~nteodosio/update-manager/+git/update-manager/+merge/455614

Fix incorrect available version for Ubuntu Pro updates in unattached case.

Can't run autopkgtests right now: https://irclogs.ubuntu.com/2023/11/15/%23ubuntu-quality.txt. But lines are <80 characters.

Test case
---------

  ua detach --assume-yes && apt install hello
  # Make sure 'apt policy hello' gives non-esm version
  ./update-manager

Select hello package and verify that "Available version" is an esm one, and not the same as the "Installed version".

Also verify that non-pro packages' "Available version" show something sensible and didn't regress.
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