0.20.0 staging

[jcapriot]

Summary

Staging for 0.20.0

PR Checklist

• If this is a work in progress PR, set as a Draft PR
• Linted my code according to the style guides.
• Added tests to verify changes to the code.
• Added necessary documentation to any new functions/classes following the
  expect style.
• Marked as ready for review (if this is was a draft PR), and converted
  to a Pull Request
• Tagged @simpeg/simpeg-developers when ready for review.

What does this implement/fix?

• Adds the release notes for version 0.20.0
• Updates deprecations and errors for the new version (make a few old functions now throw errors instead of deprecations) Also caught a few missed 0.19.0 future warnings.

[codecov]

Codecov Report

Merging #1275 (a49411f) into main (2389485) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1275      +/-   ##
==========================================
- Coverage   82.39%   82.36%   -0.03%     
==========================================
  Files         164      164              
  Lines       25098    25098              
==========================================
- Hits        20679    20673       -6     
- Misses       4419     4425       +6     

Files Changed	Coverage Δ
SimPEG/flow/richards/simulation.py	96.26% <ø> (ø)
SimPEG/regularization/__init__.py	95.00% <ø> (ø)
SimPEG/utils/io_utils/io_utils_pf.py	96.61% <ø> (ø)
SimPEG/directives/directives.py	83.09% <100.00%> (ø)
SimPEG/utils/mat_utils.py	91.45% <100.00%> (ø)

... and 1 file with indirect coverage changes