Update project references to include .csproj files instead of .proj files

[tg123]

this is likely a typo in previous PR

[codecov-commenter]

Codecov Report

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1524   +/-   ##
=========================================
  Coverage          ?   61.38%           
=========================================
  Files             ?      103           
  Lines             ?     3048           
  Branches          ?      637           
=========================================
  Hits              ?     1871           
  Misses            ?     1177           
  Partials          ?        0           

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[brendandburns]

/lgtm
/approve

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