Increase CMake version range to 3.20

[Flamefire]

Avoids deprecation warnings with CMake 3.31+

Closes #199

[nigels-com]

I was pondering if this ought to be distinct for BOOST_SUPERPROJECT_SOURCE_DIR mode and stand-alone.
In super-project mode we agree on something like 3.19 and in stand-alone mode it's a matter for the individual library.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.30%. Comparing base (7ba7e46) to head (feb955d).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #200      +/-   ##
===========================================
+ Coverage    99.10%   99.30%   +0.19%     
===========================================
  Files           34       34              
  Lines         3591     3591              
===========================================
+ Hits          3559     3566       +7     
+ Misses          32       25       -7     

see 1 file with indirect coverage changes

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