[K8s] Fix default runtime build broken by Debian Buster EOL

[mskuratowski]

Problem

The default k8s runtime build (lithops/serverless/backends/k8s/k8s.py:175 and runtime/kubernetes/Dockerfile) bases on python:*-slim-buster. Debian 10 (Buster) reached EOL in June 2024 and its APT repos moved to archive, so the generated RUN apt-get update fails with 404s and the build aborts. Every new k8s user following the documented "default runtime is built the first time you execute a function" flow hits this.

Fix

• slim-buster -> slim-bookworm (Debian 12, supported until 2028) in both places.
• Bundled Dockerfile: drop EOL Python 3.6 / 3.7 variants (no slim-bookworm tag published), add 3.11 / 3.12.
• Drive-by: typo Dockefile.default-k8s-runtime → Dockerfile.default-k8s-runtime.

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[JosepSampe]

Thanks for the contribution @mskuratowski !