[jdk8u-dev] RFR: 8331730: [8u] GHA: update sysroot for cross builds to Debian bullseye

[Zdenek Zambersky]

Updates 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](http://httpredir.debian.org/debian/dists/buster/main/) of `buster` (currently used Debian). Not sure why arches disappeared, but turns out that `buster` soon reaches end of LTS support ([2024-06-30](https://wiki.debian.org/LTS)).

As Debian `bullseye` (next version) again has all aches in [its repos](http://httpredir.debian.org/debian/dists/bullseye/main/), update solves both issues. In newer JDKs, sysroot update to bullseye was included in [JDK-8293107](https://bugs.openjdk.org/browse/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](https://bugs.openjdk.org/browse/JDK-8281096)  (Flags introduced by configure script are not passed to ADLC build). So Ubuntu update 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.

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Commit messages:
 - Update debian used GHA crossbuilds to bullseye

Changes: https://git.openjdk.org/jdk8u-dev/pull/491/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331730
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/491.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/491/head:pull/491

PR: https://git.openjdk.org/jdk8u-dev/pull/491