[ci] Replace CXX17FLAGS with CXX20FLAGS

[jdblischak]

Not important, but I noticed this use of CXX17 earlier this week and wanted to fix it before I forgot about it

xref: #3154, #3352

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.27%. Comparing base (cd1e487) to head (5f1780b).
Report is 3 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (cd1e487) and HEAD (5f1780b). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (cd1e487)	HEAD (5f1780b)
python	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3393       +/-   ##
===========================================
- Coverage   85.79%   56.27%   -29.53%     
===========================================
  Files          55       84       +29     
  Lines        6176    11525     +5349     
  Branches        0     1068     +1068     
===========================================
+ Hits         5299     6486     +1187     
- Misses        877     4663     +3786     
- Partials        0      376      +376     

Flag	Coverage Δ
libtiledbsoma	56.27% <ø> (?)
python	?

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Components	Coverage Δ
python_api	∅ <ø> (∅)
libtiledbsoma	50.91% <ø> (∅)