Normalise changelog for 2027 release

[osinjoku]

Description

Update changelog for 27.0.0 release.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

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• I have read the CONTRIBUTING document
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly
• I have added a note to CHANGELOG.md describing my changes
• My changes generate no new warnings
• Any dependent changes have been merged and published

Testing

N/A.

Additional Notes

Add any other context about the pull request here.

[Copilot]

Pull request overview

This PR prepares the changelog for the 27.0.0 release by adding release dates to recent versions and creating a new unreleased section structure.

Changes:

• Added release date (2026-01-21) for version 27.0.0
• Created new "Unreleased" section template with standard changelog categories
• Added missing release dates for versions 26.0.5, 26.0.4, and 26.0.3

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