Update vendored crates flagged by RustSec to their newest semver-compatible
releases with `cargo update -p <crate>@<version>`:
  crossbeam-epoch 0.9.18 -> 0.9.21 (RUSTSEC-2026-0204)

--- a/Cargo.lock
+++ b/Cargo.lock
@@ -466,9 +466,9 @@
 
 [[package]]
 name = "crossbeam-epoch"
-version = "0.9.18"
+version = "0.9.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5b82ac4a3c2ca9c3460964f020e1402edd5753411d7737aa39c3714ad1b5420e"
+checksum = "dc74980687109a3b14c72fd458107bf0baa1da1a1a805e178d15501ba9b86d9d"
 dependencies = [
  "crossbeam-utils",
 ]