Please remove Stretch from CI tests

The repository is no longer maintained after Buster became oldoldstable:

Reading package lists...
E: The repository 'http://deb.debian.org/debian stretch Release' does no longer have a Release file.
E: The repository 'http://deb.debian.org/debian stretch-updates Release' does no longer have a Release file.
E: The repository 'http://security.debian.org/debian-security stretch/updates Release' does no longer have a Release file.