The file which sets this option is actually part of the ubuntu-release-upgrader source package:
$ dpkg -S /etc/update-manager/release-upgrades ubuntu-release-upgrader-core: /etc/update-manager/release-upgrades
The file which sets this option is actually part of the ubuntu- release- upgrader source package:
$ dpkg -S /etc/update- manager/ release- upgrades release- upgrader- core: /etc/update- manager/ release- upgrades
ubuntu-