chore(release): bump version to 1.6.0

[quinnj]

Summary

• Bump JSON.jl from 1.5.2 to 1.6.0 for the next release.
• Includes the merged fix(parse): default bare JSON.Object keys to String #457 parser fix and feat(write): serialize complex numbers #459 Complex serialization feature.
• Intentionally excludes fix(parse): honor custom style lifts #458; that PR remains draft/open and unmerged per maintainer direction.

Verification

• git diff --check
• Registry latest confirmed as 1.5.2 before bump.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.26%. Comparing base (0e34429) to head (cf670bc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #460   +/-   ##
=======================================
  Coverage   90.26%   90.26%           
=======================================
  Files           7        7           
  Lines        1397     1397           
=======================================
  Hits         1261     1261           
  Misses        136      136           

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