refactor(dependencies): pin numpy<2 until other reqs support it

[wpbonelli]

• as recommended in NumPy 2.0 development status & announcements numpy/numpy#24300
• once required (pandas, matplotlib) and optional (e.g. scipy) dependencies support v2, a new flopy release can be made
• ci: fix optional dependency workflow #2089 originally proposed CI testing against numpy nightly build but this is not feasible until other deps support v2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ad35b8d) 73.0% compared to head (2124d29) 72.9%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2092     +/-   ##
=========================================
- Coverage     73.0%   72.9%   -0.2%     
=========================================
  Files          259     259             
  Lines        59357   57980   -1377     
=========================================
- Hits         43352   42286   -1066     
+ Misses       16005   15694    -311     

see 77 files with indirect coverage changes