Support ViennaLS 5.8.5: ship libViennaLS_GPU.so and add GpuMode.Auto

[FilipovicLado]

Adapts ViennaPS to ViennaLS 5.8.5, which moved the GPU BiCGSTAB solver into a
separate libViennaLS_GPU.so loaded at runtime.

GPU library packaging

ViennaLS no longer links ViennaLS_GPU, so it
is no longer inherited transitively. The Python build now installs the library
into the package directory, next to the extension module, where ViennaLS's
loader looks for it. Without this, GPU oxidation fails at runtime with
libViennaLS_GPU.so: cannot open shared object file.

GpuMode.Auto

Bind the new GpuMode.Auto: use the GPU when it is available, otherwise warn
and fall back to the CPU. GpuMode.Gpu keeps its "GPU or fail" behaviour.

Dependency bump

• GIT_TAG -> v5.8.5, ViennaLS>=5.8.5.
• ViennaPS 4.6.1 -> 4.6.2.

CI

• vcpkg.json disables hdf5's szip feature, removing libaec from the
  Windows build. Its only source (gitlab.dkrz.de) began rejecting all
  downloads with HTTP 429, breaking every cold Windows build. Nothing here
  reads HDF5.