[ci] Modernize OpenMPI CI images to Ubuntu 26.04 / OpenMPI 5 (fix flaky mpi_group ORTE session-dir race)

[the-hampel]

Summary

Modernize the two OpenMPI-based GitHub Actions CI images to ubuntu:26.04,
which ships OpenMPI 5.0.10 (PRRTE/PMIx), clang 22, gcc 16 and Python 3.14.

This fixes intermittent MPI-test failures in the image builds, e.g.:

26/377 Test #27: mpi_group_np4 ...***Failed
  mkdir /tmp/ompi.buildkitsandbox.0/pid.XXXX : No such file or directory
  orte_session_dir failed  (session_dir.c:107)

Root cause: OpenMPI 4's ORTE uses one per-host session directory
(/tmp/ompi.<host>.<uid>) shared across concurrent mpirun invocations. With
ctest -j, two MPI tests run at once; one's cleanup removes the shared parent
while another is creating its pid.* subdir → a race that aborts a random MPI
test at init. It's nondeterministic (one image happened to pass, the other
failed on the identical test). OpenMPI 5 replaces ORTE with PRRTE and no longer
has this shared-session-dir race, so ctest can stay parallel.

Changes (both github_ci_dockerfile and openmpi_dockerfile)

• FROM ubuntu:24.04 → ubuntu:26.04
• ARG LLVM 20 → 22
• Python 3.12 → 3.14 (26.04 default); hardcoded python3.12 paths parametrized
  via ${PYTHON_VERSION} so they don't drift again
• Oversubscribe env: OMPI_MCA_rmaps_base_oversubscribe=yes was removed in
  OpenMPI 5 (silently ignored → np>cores fails "not enough slots"). Replaced
  with the PRRTE equivalent PRTE_MCA_rmaps_default_mapping_policy=:oversubscribe.
• pip install gains --break-system-packages (PEP 668 on 26.04; harmless with --target)
• github_ci builds the stack with gcc; bumped gcc-14 → gcc-16 because
  clang-22 (used for clair) selects the highest gcc install dir for libstdc++
  (dir 16 on 26.04) — installing gcc-16 populates it so clair links without a pin.
• openmpi builds with clang + -stdlib=libc++, but clair is still built with
  libstdc++; added g++-16 for the same reason.

mpich_dockerfile is intentionally left unchanged (MPICH/Hydra has no ORTE
session-dir race).

Validation

Built locally on arm64 under x86 emulation:

• github_ci image builds through the full triqs compile; 376/377 tests pass,
  and crucially mpi_group_np2 and mpi_group_np4 both pass on OpenMPI 5 — the
  previously flaky tests. The one local failure (nda_basic_array_and_view, SIGTRAP
  at 0.05 s, isolated among all passing neighbours) is a suspected qemu-emulation
  artifact and should be confirmed on the native amd64 runner here.
• openmpi image can't be fully built under emulation (qemu ENOSYS in
  wannier90's tar --xform, unrelated to these changes), but its 26.04 package set
  resolves, clang-22 -stdlib=libc++ compiles/links, and the PRRTE oversubscribe
  env works.

Note: the Jenkins CI continues to run against Ubuntu 24.04, so this keeps
cross-distro coverage (24.04 on Jenkins, 26.04 on the GitHub images).