ci: use native ARM64 runners instead of QEMU for multi-arch builds (#842)

## Summary

Replace QEMU-emulated cross-compilation with native per-architecture
builders (`ubuntu-latest` for amd64, `ubuntu-24.04-arm` for arm64) and a
manifest merge step. This eliminates SIGILL crashes caused by QEMU's
incomplete instruction emulation and builds both arches in parallel for
faster CI.

## Changes

- Split `build-and-push` into `build` (per-arch native runners) +
`merge-manifests` (creates multi-arch manifest via `docker buildx
imagetools create`)
- Dynamic matrix: only build changed components, skip unchanged entirely
- Applied to both `components-build-deploy.yml` (stage) and
`prod-release-deploy.yaml` (production)
- Removed stale `public-api` references

## Speed gains

- ARM64 builds run at native speed instead of ~5-10x slower QEMU
emulation
- Both architectures build in parallel on separate runners
- Eliminates SIGILL crashes from QEMU's incomplete instruction emulation

**Jira:**
[RHOAIENG-52962](https://issues.redhat.com/browse/RHOAIENG-52962)

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: maskarb