demos: bump cmake version required to 3.15

[gdamore]

This fixes problems finding the threads library. There may still be problems building these with TLS libraries added, as those dependencies may not be satisfied if the library lacks a cmake configuration file.

fixes #

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.62%. Comparing base (b5462f6) to head (60ff324).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2031      +/-   ##
==========================================
- Coverage   82.69%   82.62%   -0.07%     
==========================================
  Files          95       95              
  Lines       23656    23656              
  Branches     3044     3044              
==========================================
- Hits        19562    19546      -16     
- Misses       4023     4036      +13     
- Partials       71       74       +3     

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