Revert "Downgrade to curve25519-dalek v4.1.3 (#7079)"

[apfitzge]

Problem

This manual downgrade did not properly revert the original commit.

Summary of Changes

This reverts commit d1e9a3e.
This will be followed up by a revert of #6914.

Fixes #

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.2%. Comparing base (86d14f6) to head (3768d88).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #7150     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         853      853             
  Lines      373926   374002     +76     
=========================================
+ Hits       311358   311367      +9     
- Misses      62568    62635     +67     

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