remove debian jessie as the docker image used for testing is no longer valid

[atoulme]

The current Docker image's GPG keyring has expired and is no longer able to authenticate packages.
The Docker image is over 2 years old, and Jessie is no longer maintained since 2020.

#5 12.64 WARNING: The following packages cannot be authenticated!
#5 12.64   libgmp10 libnettle4 libhogweed2 libp11-kit0 libkeyutils1 python-minimal
#5 12.64   mime-support libpython-stdlib python init-system-helpers libpsl0
#5 12.64 E: There are problems and -y was used without --force-yes
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executor failed running [/bin/sh -c apt-get update &&    apt-get install -yq ca-certificates procps wget apt-transport-https init-system-helpers curl python gnupg]: exit code: 100