Bump crossbeam-epoch from 0.9.18 to 0.9.20

[cakebaker]

Fixes #485

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.05%. Comparing base (30607c4) to head (d5b15a7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #486   +/-   ##
=======================================
  Coverage   83.05%   83.05%           
=======================================
  Files          13       13           
  Lines        6120     6120           
  Branches      358      358           
=======================================
  Hits         5083     5083           
  Misses       1034     1034           
  Partials        3        3           

Flag	Coverage Δ
macos_latest	83.76% <ø> (ø)
ubuntu_latest	83.84% <ø> (ø)
windows_latest	0.00% <ø> (ø)

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