📝 Update changelog after release of v3.4.1

[denialhaag]

Description

This PR updates the changelog after the release of v3.4.1 from the stable v3.x branch.

Checklist:

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

[coderabbitai]

📝 Walkthrough

Summary by CodeRabbit

• Documentation

  • Released version 3.4.1 with updated dependencies including nanobind, Boost, spdlog, and QDMI.

• Changed

  • MLIR now built by default for C++ library builds.

• Removed

  • Density matrix support from MQT Core DD.
  • Datastructures sublibrary.

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Walkthrough

The CHANGELOG.md is updated to document version 3.4.1 release, including dependency updates (nanobind, Boost, spdlog, QDMI), removal of density matrix support from MQT Core DD, removal of the datastructures sublibrary, and MLIR building by default for C++ library builds.

Changes

Cohort / File(s)	Summary
Changelog Release Documentation CHANGELOG.md	Added new version section [3.4.1] with dependency updates and reformatted existing entries. Updated Unreleased section to include MLIR default build for C++, removal of density matrix support, and datastructures sublibrary. Adjusted PR/link references and version transition lines.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• 🔖 Update changelog with v3.4.0 release #1434: Modifies the same CHANGELOG.md entries for MLIR default build configuration in C++ library builds.
• 🔥 remove dedicated MLIR option for reusable-change-detection.yml and merge with C++ checks workflows#300: Merges MLIR detection into C++ workflows, implementing the feature documented in this changelog update.
• 🔖 Prepare release of v3.4.0 qmap#805: Performs similar changelog maintenance for release preparation and version bump coordination.

Suggested reviewers

• burgholzer

Poem

🐰 ✨ Versions hop and changelogs grow,
MLIR now defaults, as far as we know,
From 3.4.0 to 3.4.1 we bound,
Datastructures gone, new deps we've found! 📝

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: updating the changelog after releasing v3.4.1, which aligns with the file modifications and PR objectives.
Description check	✅ Passed	The description provides a clear summary of the change and appropriately marks non-applicable checklist items as struck through, which is acceptable for documentation-only changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch update-changelog

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