fix(rasters): accommodate rasterio>=1.5

[wpbonelli]

Pass adjust=False to rasterio.plot.show() unless the caller sets it True. Accommodate the default switch in rasterio/rasterio#3171. Looks like the default may be switched back rasterio/rasterio#3549, but in any case this guarantees the right behavior by default, and a caller choosing to pass adjust=True can set vmin/vmax accordingly.

Close #2780

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.6%. Comparing base (556c088) to head (25e7b89).
⚠️ Report is 182 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/rasters.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2781      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132734    +8599     
===========================================
+ Hits         68947    96367   +27420     
+ Misses       55188    36367   -18821     

Files with missing lines	Coverage Δ
flopy/utils/rasters.py	70.9% <0.0%> (-3.1%)	⬇️

... and 579 files with indirect coverage changes

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