Draft: Release 5.0

Closes #51, #53, #57

• Update to OTB 9.1
• Move back TF build env and OTB build flags into Dockerfile
• Update bazel build to latest specs (Clang, auto deps download HERMETIC_CUDA)
• Use TF 2.18 + official TF wheels setup (--config release_*)
• Unified build process and base image for CPU and GPU images
• Add build arg CUDA_CC for compute capabilities
• Drop INLINE_CACHE, use dedicated repository cache => reduce images size to half
• Move python installation to venv (required for future build py312)
• Install dev / tests packages in dev image to speedup pipelines
• Run release CI only on tags
• Update core module to Keras3 (example models used in failing tests)
• Update layers module to Keras3 ops
• Fix decloud test
• Move packaging spec to pyproject.toml ( #41 )