Clean up META.json slightly

[jeltz]

This cleans up two things:

• Use shorthand for the licence since it is a known one
• Skip the optional generated_by field which does not add any value

Link to documentation: https://pgxn.org/spec/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.32%. Comparing base (44b9bb7) to head (1d915cd).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #643   +/-   ##
=======================================
  Coverage   86.32%   86.32%           
=======================================
  Files           3        3           
  Lines        1346     1346           
  Branches      217      217           
=======================================
  Hits         1162     1162           
  Misses         87       87           
  Partials       97       97           

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