chore(deps): update crossbeam-epoch to 0.9.20

[gkamathe]

What does this PR try to resolve?

cargo audit showed a crossbeam-epoch finding [1]

$ cargo audit 
    Fetching advisory database from `https://github.com/RustSec/advisory-db.git`
      Loaded 1239 security advisories (from /root/.cargo/advisory-db)
    Updating crates.io index
    Scanning Cargo.lock for vulnerabilities (550 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

[1] https://rustsec.org/advisories/RUSTSEC-2026-0204

How to test and review this PR?

Ran cargo build and cargo test post the update, both worked fine, no issues found

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