docs(changelog): update DifferentiationInterface changelog for 0.7.21

[gdalle-bot]

This PR was opened by a coding agent, please disregard it until @gdalle has reviewed it

Summary

Updates the DifferentiationInterface changelog to document the user-facing changes merged since the 0.7.20 release:

• Update HyperHessians extension for HyperHessians v0.3, forwarding the new simd and jet backend options (update HyperHessians extension for 0.3 #1053)
• Make FiniteDiff prep non-allocating with static arrays (fix: make FiniteDiff prep non-allocating with static arrays #1019) (already listed)
• Avoid duplicate function calls with FiniteDiff (fix: avoid duplicate function calls with FiniteDiff #1061)

Test-only and CI-only changes (JET/Julia 1.12 fixes, benchmark workflow split, README fixes, pre-commit hook additions, and the Mooncake HVP feature that was added then reverted) were intentionally left out as not user-facing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.40%. Comparing base (a7892c6) to head (378a9f7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1064   +/-   ##
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  Coverage   97.40%   97.40%           
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  Files         143      143           
  Lines        8299     8299           
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  Hits         8084     8084           
  Misses        215      215           

Flag	Coverage Δ
DI	97.91% <ø> (ø)
DIT	96.04% <ø> (ø)

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