Downgrade OpenCV 4.5.1 msg to WARNING

[ferdnyc]

I misread the CMake docs, and didn't realize that SEND_ERROR message severity would still register as a failed config at the end of the run. So, OpenCV 4.5.1 is still killing our Ubuntu Impish PPA builds.

Downgrade to WARNING to let them succeed.

[codecov]

Codecov Report

Merging #664 (3a4f2ae) into develop (acda566) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #664   +/-   ##
========================================
  Coverage    52.42%   52.42%           
========================================
  Files          151      151           
  Lines        12346    12346           
========================================
  Hits          6473     6473           
  Misses        5873     5873           

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