Add upcoming release notes to CHANGELOG

[Copilot]

This updates the changelog for the upcoming release by capturing the unreleased work merged after 7.10.0. The new entry summarizes the recent bug fixes, type-system improvements, and PHP 8+ maintenance changes in a form ready for release prep.

• Upcoming release entry

  • adds a new Upcoming release section at the top of CHANGELOG.md
  • keeps the existing changelog structure intact so unreleased notes are easy to promote into a tagged release

• Release-note coverage

  • documents the average() fix for non-numeric values on modern PHP
  • documents the changeKeyCase() Unicode behavior fix across PHP 8.0–8.5
  • summarizes the native property type-checking and array-shape regression work
  • captures the PHPStan ignore cleanup and PHP 8.0+ docs/CI refresh

• Example

  ### Upcoming release
  
  - fix `average()` so non-numeric values no longer error on modern PHP versions
  - make `changeKeyCase()` Unicode case conversion deterministic across PHP 8.0–8.5
  - strengthen native property type checks, array-shape contracts, and regression coverage across Json mapper and collection helpers
  - remove stale PHP 8-only compatibility branches, clean up PHPStan ignores, and refresh the PHP 8.0+ docs/CI matrix

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This change is 

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[coveralls]

coverage: 92.372%. remained the same — copilot/prepare-changelog-release-notes into master