Update GHA to use Debian bullseye for cross build sysroot. Motivation for this are broken builds on s390x and ppc64le in recent test runs. This is due to disappearance of s390x and ppc64le arches in repos of buster (currently used Debian). Not sure why arches disappeared, but turns out that `buster` soon reaches end of LTS support (2024-06-30).

As Debian bullseye (next version) again has all aches in its repos, update solves both issues. In newer JDK sysroot update to bullseye was done as part of JDK-8293107 (GHA: Bump to Ubuntu 22.04). I have tried Ubuntu update in GHA as well, but it failed to build on some platforms (aarch64, ppc64le) with errors such as:

/home/runner/work/jdk8u-dev/jdk8u-dev/jdk/hotspot/src/share/vm/adlc/arena.cpp:82:19: error: ISO C++17 does not allow ‘register’ storage class specifier [-Werror=register]
   82 | register Chunk *k = _first;
      | ^

Seems to be JDK-8281096 (Flags introduced by configure script are not passed to ADLC build). So Ubuntu upgrade is blocked by that (and there seem to be some complications there). That's why I would like to do this separately from Ubuntu upgrade.