Update wolfSSL version in ML-KEM and Thread tests

[ejohnstown]

1. Update WOLFSSL_REF to v5.9.1-stable in the ML-KEM and single-thread tests (interop-mlkem.yml, singlethread-check.yml).
2. Drop kyber.yml; its coverage is a strict subset of interop-mlkem.yml. Fold its push and workflow_dispatch triggers into interop-mlkem.yml so ML-KEM still runs on master/release pushes and via manual dispatch.
3. Drop liboqs from the ML-KEM test. It isn't in configure.ac any more.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #984

No scan targets match the changed files in this PR. Review skipped.

[Copilot]

Pull request overview

This PR updates the GitHub Actions CI coverage for ML-KEM and single-thread testing by pinning wolfSSL to a newer stable tag, consolidating redundant ML-KEM workflows, and removing liboqs usage from the ML-KEM CI path.

Changes:

• Bump WOLFSSL_REF to v5.9.1-stable for ML-KEM interop and single-thread CI workflows.
• Remove kyber.yml and fold its push/workflow_dispatch triggers into interop-mlkem.yml.
• Simplify the ML-KEM interop workflow to use a single wolfSSL ref instead of a wolfSSL version matrix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/singlethread-check.yml	Pins single-thread CI to wolfSSL v5.9.1-stable.
.github/workflows/kyber.yml	Removes the redundant ML-KEM workflow (coverage now handled by interop-mlkem.yml).
.github/workflows/interop-mlkem.yml	Adds push/workflow_dispatch triggers, pins wolfSSL to v5.9.1-stable, and removes the prior wolfSSL version matrix.

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