ONNX Opset / Version Bump Checklist for ONNX Runtime

Repeatable process for upgrading the ONNX dependency in ONNX Runtime. Recurs on every ONNX release. Canonical reference PRs: #27601 (ONNX 1.21, incremental rc1→formal — best example), #26579 (1.20.1), #25678 (1.19). See also docs/How_To_Update_ONNX_Dev_Notes.md.

0. Strategy: RC → formal (incremental)

ONNX partner-validates each release candidate against ORT before publishing the formal release — that is the whole point of the integration issue. When the issue points at a rel-X.Y.0 branch:

1. Integrate the RC first. Pin by the release-branch HEAD commit (the vX.Y.0rcN git tag usually does not exist yet). Run the full build + tests; file ONNX bugs upstream (tag the ONNX release manager) for any ONNX-side defects found.
2. Re-pin per RC (rc2, rc3, …). Usually only the Group-A version-plumbing files change. Drop any onnx.patch hunks that ONNX has merged upstream in the new RC.
3. Re-pin to the formal tag vX.Y.0 once the GitHub Release is published. Note the release can sit as a draft (no git tag, git/ref/tags/vX.Y.0 → 404) for a while.