rust: Bump crossbeam-epoch to 0.9.20

[martintomazic]

Fixes:

Run cargo audit
info: syncing channel updates for nightly-2026-02-11-x86_64-unknown-linux-gnu
info: latest update on 2026-02-11 for version 1.95.0-nightly (9e79395f9 2026-02-10)
info: downloading 7 components
    Fetching advisory database from `[https://github.com/RustSec/advisory-db.git`](https://github.com/RustSec/advisory-db.git%60)
      Loaded 1160 security advisories (from /home/runner/.cargo/advisory-db)
    Updating crates.io index
    Scanning Cargo.lock for vulnerabilities (780 crate dependencies)
Crate:     crossbeam-epoch
Version:   0.9.18
Title:     Invalid pointer dereference in `fmt::Pointer` impl for `Atomic` and `Shared` when the underlying pointer is invalid
Date:      2026-07-06
ID:        RUSTSEC-2026-0204
URL:       https://rustsec.org/advisories/RUSTSEC-2026-0204
Solution:  Upgrade to >=0.9.20

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✅ Deploy Preview for oasisprotocol-oasis-sdk canceled.

Name	Link
🔨 Latest commit	c715a24
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.23%. Comparing base (47c935c) to head (c715a24).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2488      +/-   ##
==========================================
- Coverage   51.23%   51.23%   -0.01%     
==========================================
  Files         237      237              
  Lines       17294    17294              
==========================================
- Hits         8861     8860       -1     
- Misses       8373     8374       +1     
  Partials       60       60              

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[peternose]

Bump in Oasis Core also until a new version of crossbeam is released.

[martintomazic]

Bump in Oasis Core also until a new version of crossbeam is released.

Was already done but forgot to update sdk at that time :)
oasisprotocol/oasis-core@cd87e3b

[peternose]

Was already done

Nice. Was looking an old branch 🤦