Unify digest field names across the project

[JSCU-CNI]

Currently the record field names digest and digests are used interchangeably, which is inconsistent. This PR replaces all occurrences of digests with digest.

This eases the use of a strict dissect Elasticsearch mapping.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.24%. Comparing base (6770095) to head (1a0570e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #982      +/-   ##
==========================================
- Coverage   77.72%   76.24%   -1.48%     
==========================================
  Files         326      326              
  Lines       28571    28571              
==========================================
- Hits        22206    21784     -422     
- Misses       6365     6787     +422     

Flag	Coverage Δ
unittests	76.24% <ø> (-1.48%)	⬇️

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