Finalize CHANGELOG for v1.6.0

[pawelMSoftware]

Summary

Closes out CHANGELOG.md's "Unreleased" section for the referee association normalization work
(data/football_associations.json, referees.json's association → association_id, 18 new
countries.json records) merged in #27, under a ## [1.6.0] heading. Pure record-keeping — no
data/, docs (other than CHANGELOG.md itself), or test changes.

Type of change

• Data correction (a value in data/ was wrong)
• New tournament / new records
• Documentation
• Test suite

Source

Not applicable — no data or fact changed, just the changelog heading.

Checklist

• One logical change per PR (see docs/CONTRIBUTING.md)
• composer test passes locally (119 passed, 24994 assertions — unaffected by this change)
• Record id values were not regenerated (no data touched)
• docs/CHANGELOG.md updated under "Unreleased" (this PR is that update)
• docs/CONVENTIONS.md / docs/DATA_MODEL.md — not applicable, no rule changed

Link: https://github.com/pawelMSoftware/world-cup-data/pull/new/docs/finalize-1.6.0-changelog