[SCHEMA] Conform the schema/rules files consistently

[ericearl]

1. Made a lot of edits to lowercase all the OPTIONAL, REQUIRED, RECOMMENDED, PROHIBITED, DEPRECATED, and RECOMMENDED values.
2. Some edits were made to standardize the way issues appeared with "code", "message", and "level" (error or warning) nested under "issue" instead of "name" and "description" un-nested.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.50%. Comparing base (1f6e4d2) to head (ea25b43).

Additional details and impacted files

@@              Coverage Diff               @@
##           schema-sprint    #1078   +/-   ##
==============================================
  Coverage          71.50%   71.50%           
==============================================
  Files                  9        9           
  Lines                930      930           
==============================================
  Hits                 665      665           
  Misses               265      265           

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