refactor: fixes for numpy-2.0 deprecation warnings, require numpy>=1.20.3

[mwtoews]

This PR resolves several deprecation warnings from NumPy 2.0 (xref #2153):

• np.in1d() → np.isin()
• "a16" → "S16" to describe 16-length string of bytes
• The numpy.core.records module is private, so use numpy.rec instead, which seems to work with older versions

This also bumps the minimum version of numpy from 1.15.0 to 1.20.3, since this is the minimum for pandas 2.0.0, thus the minimum versions were incompatible.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 66.7%. Comparing base (e2a85a3) to head (3ec1915).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2237     +/-   ##
=========================================
- Coverage     70.8%   66.7%   -4.1%     
=========================================
  Files          294     294             
  Lines        59026   58888    -138     
=========================================
- Hits         41832   39331   -2501     
- Misses       17194   19557   +2363     

Files	Coverage Δ
flopy/modflow/mfsfr2.py	86.7% <100.0%> (-0.7%)	⬇️
flopy/plot/plotutil.py	55.7% <100.0%> (-0.4%)	⬇️
flopy/utils/binaryfile.py	81.9% <100.0%> (+0.4%)	⬆️
flopy/utils/datafile.py	74.7% <ø> (+1.4%)	⬆️
flopy/utils/modpathfile.py	87.9% <100.0%> (-0.6%)	⬇️
flopy/utils/recarray_utils.py	94.4% <ø> (ø)
flopy/utils/util_list.py	75.1% <100.0%> (-0.4%)	⬇️
flopy/modflow/mfgage.py	89.0% <0.0%> (ø)
flopy/utils/zonbud.py	67.0% <88.8%> (+0.1%)	⬆️
flopy/modflow/mffhb.py	63.2% <0.0%> (-0.2%)	⬇️
... and 1 more

... and 84 files with indirect coverage changes