Changes

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.

Related: OS#4969
Change-Id: Ib3bdfa3bec8f8ef42c55ca61cdee8fbca923874f

Switch ttcn3-* master tests from Debian stretch to bullseys

So far we were using ancient Debian 9 (stretch) for our IUT
containers.  Let's upgrade that to Debian 11 (bullseye).

Related: OS#4969
Change-Id: Ic6bece9cb695e6eccfcb1e83fdbf1048724a3cf9

nplab: Switch from stretch to buster

The test suites require guile-2.0 so we have to stay with buster
and cannot upgrade to bullseye (guile-3.0 only).

Related: OS#4969
Change-Id: I30c05efbc6c7a21cad71b207e723ef958f1ac9be

sctp-test: Upgrade to bullseye

Change-Id: I681802fd779530878c2d2358ebf90b8e2adb4870