Allow multihashes with DBL_SHA2_256 code

[gammazero]

The double hash multicodec code is not dedicated to encrypted records. Therefore we cannot implicitly assume that it represents one.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 55.69%. Comparing base (c72e65b) to head (be5a34b).

Files	Patch %	Lines
store/dhstore/dhstore.go	0.00%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
+ Coverage   55.50%   55.69%   +0.19%     
==========================================
  Files          20       20              
  Lines        2908     2898      -10     
==========================================
  Hits         1614     1614              
+ Misses       1067     1057      -10     
  Partials      227      227              

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