docker: add multi-arch builds and update base images

[andreialecu]

Summary

Adds multi-architecture Docker builds (amd64 + arm64) and updates Docker images to modern Ubuntu/Jammy bases. This resolves ARM64 build/pull issues and deprecates CentOS-based Dockerfiles.

Closes #5421.

What changed

• CI/CD: GitHub Actions now uses Buildx + QEMU and builds/pushes multi-arch images with GHA cache for countly/countly-server, countly/api, countly/frontend, and countly/countly-core.
• Base images: Dockerfile and Dockerfile-core move from phusion/baseimage:focal to jammy; MongoDB apt repo updated to Jammy.
• API/Frontend Dockerfiles: switch to distro Python + tini packages (drop manual Python 3.8 and tini downloads), simplify cleanup, and remove CentOS Dockerfiles.
• Docs: README updated with multi-arch support and CentOS Docker deprecation notice.

Why

ARM64 users were blocked by missing multi-arch images and older base images. This update ensures ARM64 images are built and published and aligns the Dockerfiles with supported OS bases.

Testing

• docker compose up -d on an ARM64 Apple Silicon machine
• API and frontend pings return success (via nginx and direct service checks)