fix(wasm-utxo): pin crossbeam-epoch to >=0.9.20 (RUSTSEC-2026-0204)

[Ranjna-G]

Summary

Pins crossbeam-epoch to >=0.9.20 to fix a security advisory that is currently failing cargo deny check in CI.

Changes

• Added crossbeam-epoch = ">=0.9.20" as a direct dependency in wasm-utxo/Cargo.toml to force the resolver to a safe version
• Updated Cargo.lock to reflect the resolved version (0.9.18 → 0.9.20)

Advisory

RUSTSEC-2026-0204: Invalid pointer dereference in fmt::Pointer impl for Atomic and Shared when the underlying pointer is invalid (e.g. created with Atomic::null or Shared::null).

• Fix: Do not dereference pointer in fmt::Pointer crossbeam-rs/crossbeam#1276
• Solution: upgrade to >=0.9.20

Test Plan

• cargo deny check should pass in CI