api/build_runner.sh uses deprecated debian bullseye-backports

[aagbsn]

When running "integration_test" on my fork of ooni/backend, the test fails with this error:

 failed to solve: process "/bin/sh -c ./build_runner.sh" did not complete successfully: exit code: 100

0.478 Get:7 http://deb.debian.org/debian-security bullseye-security/main amd64 Packages [420 kB]
0.539 Get:8 http://deb.debian.org/debian bullseye-updates/main amd64 Packages [18.8 kB]

1.754 E: The repository 'http://deb.debian.org/debian bullseye-backports Release' does not have a Release file.
------
make[1]: *** [Makefile:26: .state/docker-build] Error 1
make: *** [Makefile:34: build] Error 2
make[1]: Leaving directory '/home/runner/work/backend/backend/api'
Error: Process completed with exit code 2.

See: https://github.com/aagbsn/backend/actions/runs/18866436670/job/53834883407

From https://backports.debian.org/Instructions/

When a new Debian release is made, the previous stable suite becomes oldstable. While oldstable releases may have [Long Term Support](https://wiki.debian.org/LTS), oldstable-backports are only made available for a period of one year after a new Debian stable release has been made. For example, Debian 12 'bookworm' was released on June 10th 2023, so bullseye-backports for Debian 11 'bullseye' was [closed](https://lists.debian.org/debian-backports-announce/2024/07/msg00000.html) on June 10th 2024. Packages in oldstable-backports must track the versions in stable (or stable/updates for security fixes).

Suggestion: switch to debian stable