Follow c2sp.org/tlog-witness#280 changes

[AlCutter]

This PR updates the witness package with the recent changes in C2SP/C2SP#280.

Now, root hashes for zero-size checkpoints must be as specified in RFC6962, and 422 is used for root mismatch as well as invalid proof (both are just special cases of an inconsistent request).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.40%. Comparing base (994001e) to head (525981c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
+ Coverage   25.18%   25.40%   +0.21%     
==========================================
  Files          28       28              
  Lines        2045     2047       +2     
==========================================
+ Hits          515      520       +5     
+ Misses       1417     1413       -4     
- Partials      113      114       +1     

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