Update settings for new everett release

[robhudson]

Also:

• remove default of parser=str
• tries to be consistent on boolean strings
• Adds the new ChoiceOf where appropriate
• Use ListOf(..., allow_empty=False) where appropriate
• Remove unused settings
• Tweak logic around markus logging

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.65%. Comparing base (d203fae) to head (af6aac2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15418      +/-   ##
==========================================
- Coverage   78.68%   78.65%   -0.03%     
==========================================
  Files         156      156              
  Lines        8171     8163       -8     
==========================================
- Hits         6429     6421       -8     
  Misses       1742     1742              

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