Fix jobs for older rubies

The jobs for ruby 2.1 and 2.2 began to fail, because apparently the repository "jessie-updates" doesn't exist anymore.

Can we fix this easily? If not, I'd suggest to maybe drop or disable the pipelines for these ruby versions. They will be dropped for schleuder 4.0 anyway.

Example of failed job: https://0xacab.org/schleuder/schleuder/-/jobs/92108

W: Failed to fetch http://deb.debian.org/debian/dists/jessie-updates/main/binary-amd64/Packages  404  Not Found