titan: move from debian-stetch-titan to debian-bullseye-titan
So far we were executing all our TTCN-3 tests from a container image with Debian stretch (9) plus a custom more recent eclipse-titan package from the osmocom feed.
Let's update the container base OS from stretch (9) to bullseye (11) while using the same packaged eclipse-titan version (8.0.0) for running the tests. So this should be a low-risk change, as titan runtime remains identical.
I've executed all test suites locally and couldn't see any regressions.
