chore(deps): bump crossbeam-epoch to 0.9.20 (RUSTSEC-2026-0204)

[rakita]

Fixes the deny CI failure seen on #3803 (and any other recent runs).

A new advisory RUSTSEC-2026-0204 was published against crossbeam-epoch v0.9.18 (invalid pointer dereference in the fmt::Pointer impl for Atomic/Shared). It reaches the workspace only through the dev-dependency chain codspeed-criterion-compat → rayon → rayon-core → crossbeam-deque → crossbeam-epoch.

Fix per the advisory: cargo update -p crossbeam-epoch (0.9.18 → 0.9.20). cargo deny check advisories passes locally with this change.

[codspeed-hq]

Merging this PR will not alter performance

✅ 181 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing update-crossbeam-epoch (17b302e) with main (4c5cc3a)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩