Align BN_bn2hex behavior with OpenSSL

[samuel40791765]

Description of changes:

OpenSSL's historically printed BN_bn2hex with upper case letters, as opposed to our original upstream predecessors. We haven't had a concrete ask to change this until recently: ruby/openssl#833.
As most of the open source community and our integration targets still depend on OpenSSL as their main libcrypto dependency, we should consider making the minor adjustment to minimize churn for us and our consumers.

Call-outs:

N/A

Testing:

Tweaks to existing tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.92%. Comparing base (94daeb2) to head (3f8bbe2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2122      +/-   ##
==========================================
- Coverage   78.92%   78.92%   -0.01%     
==========================================
  Files         610      610              
  Lines      105137   105137              
  Branches    14911    14910       -1     
==========================================
- Hits        82983    82976       -7     
- Misses      21503    21510       +7     
  Partials      651      651              

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