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.
Related: OS#4969
Change-Id: Ib3bdfa3bec8f8ef42c55ca61cdee8fbca923874f