backstage release v0.137.2

CI: cap the release image build to linux/amd64. The devctl-generated config built a multi-arch (amd64+arm64) image, but the arm64 cross-build of the backend’s native modules (isolated-vm, better-sqlite3, keytar) runs under QEMU emulation and silently exceeds the orb’s 20m no-output timeout, which blocked the tag pipeline (and therefore the chart catalog push) on v0.137.1. Every prior release was single-arch amd64 and the management clusters are amd64, so arm64 is unused. See ./docs/releases/v0.137.2-changelog.md for more information.