Update changelog for v1.21.0

[halspang]

Summary

What changed?

Updated changelog for new release.

Why is this change needed?

Have to keep the changelog up to date

Issues / work items

• Resolves #
• Related #

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Project checklist

• Release notes are not required for the next release
  • Otherwise: Notes added to release_notes.md
• Backport is not required
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• All required tests have been added/updated (unit tests, E2E tests)
• Breaking change?
  • If yes:
    • Impact:
    • Migration guidance:

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AI-assisted code disclosure (required)

Was an AI tool used? (select one)

• No
• Yes, AI helped write parts of this PR (e.g., GitHub Copilot)
• Yes, an AI agent generated most of this PR

If AI was used:

• Tool(s): Copilot CLI
• AI-assisted areas/files: changelog.md
• What you changed after AI output: Fixed header

AI verification (required if AI was used):

• I understand the code and can explain it
• I verified referenced APIs/types exist and are correct
• I reviewed edge cases/failure paths (timeouts, retries, cancellation, exceptions)
• I reviewed concurrency/async behavior
• I checked for unintended breaking or behavior changes

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Notes for reviewers

• N/A

[Copilot]

Pull request overview

Updates the repository changelog to reflect the v1.21.0 release.

Changes:

• Added an Unreleased section at the top of the changelog.
• Added a v1.21.0 section with the release entries and PR links.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.