refactor(expired deprecation): raise AttributeError with to_shapefile

[mwtoews]

This PR changes deprecated .to_shapefile() functions to raise AttributeError with a helpful message.

These are the affected functions:

• flopy.utils.util_list.MfList.to_shapefile
• flopy.pakbase.Package.to_shapefile
• flopy.mbase.BaseModel.to_shapefile

While these functions have been deprecated since 3.2.4 released in 2016 (changelog), they were never documented as such. Thus a helpful exception message and cross-referenced docstring is used instead of removing these functions.

---

Xref to #1884 where a similar approach was used.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.8%. Comparing base (a49b613) to head (483c493).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2200     +/-   ##
=========================================
+ Coverage     66.2%   69.8%   +3.5%     
=========================================
  Files          294     294             
  Lines        58856   59012    +156     
=========================================
+ Hits         39009   41224   +2215     
+ Misses       19847   17788   -2059     

Files	Coverage Δ
flopy/mbase.py	69.9% <100.0%> (+0.3%)	⬆️
flopy/pakbase.py	81.9% <100.0%> (+1.0%)	⬆️
flopy/utils/util_list.py	73.9% <100.0%> (+1.1%)	⬆️

... and 208 files with indirect coverage changes

[wpbonelli]

Thanks @mwtoews