Restore the ability to bootstrap a 4.4 system

[fbartels]

Please make sure you considered the following things

• I read the contribution guidelines.
• I read the code of conduct.
• I created a bug report in the Univention Bugzilla.
• I will add a bugzilla comment about this pull request.

Link to the issue in Bugzilla

https://forge.univention.org/bugzilla/show_bug.cgi?id=

Description of the changes

Please describe the changes with a few sentences.

• https://help.univention.com/t/ucs-4-x-apt-key-expired-no-package-installation-update-app-installation-is-possible/18116
• The repo signing key for the 4.4 repo has expired and the repos have since then been signed with the 5.0 repo signing key
• correct the repo signing key in bootstrap information to be able to bootstrap a 4.4 system

[CLAassistant]

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