🔖 Prepare release of v2.1.0 (#683)

## Description

This PR prepares the release of v2.1.0.

## Checklist:

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

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Added 2.1.0 release notes with upgrade guidance and a dated entry
(2025-10-15).
* Announced support for Qiskit IfElse and Python 3.14 testing; noted
end-of-support for Python 3.9.
* Updated comparison and release links to reference the 2.1.0 tag and
refreshed Unreleased section.
  * Harmonized formatting and clarified the supported Python versions.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: denialhaag

CHANGELOG.md

@@ -9,6 +9,10 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 ## [Unreleased]
 
+## [2.1.0] - 2025-10-15
+
+_If you are upgrading: please see [`UPGRADING.md`](UPGRADING.md#210)._
+
 ### Added
 
 - ✨ Add support for Qiskit's `IfElse` operations ([#681]) ([**@denialhaag**])
@@ -64,7 +68,8 @@ _📚 Refer to the [GitHub Release Notes] for previous changelogs._
 
 <!-- Version links -->
 
-[unreleased]: https://github.com/munich-quantum-toolkit/ddsim/compare/v2.0.0...HEAD
+[unreleased]: https://github.com/munich-quantum-toolkit/ddsim/compare/v2.1.0...HEAD
+[2.1.0]: https://github.com/munich-quantum-toolkit/ddsim/releases/tag/v2.1.0
 [2.0.0]: https://github.com/munich-quantum-toolkit/ddsim/releases/tag/v2.0.0
 [1.24.0]: https://github.com/munich-quantum-toolkit/ddsim/releases/tag/v1.24.0
 

UPGRADING.md

@@ -4,6 +4,8 @@ This document describes breaking changes and how to upgrade. For a complete list
 
 ## [Unreleased]
 
+## [2.1.0]
+
 ### End of support for Python 3.9
 
 Starting with this release, MQT DDSIM no longer supports Python 3.9.
@@ -65,8 +67,9 @@ The default compilers of our test systems support all relevant features of the s
 
 <!-- Version links -->
 
-[unreleased]: https://github.com/munich-quantum-toolkit/qmap/compare/v2.0.0...HEAD
-[2.0.0]: https://github.com/munich-quantum-toolkit/qmap/compare/v1.24.0...v2.0.0
+[unreleased]: https://github.com/munich-quantum-toolkit/ddsim/compare/v2.1.0...HEAD
+[2.1.0]: https://github.com/munich-quantum-toolkit/ddsim/compare/v2.0.0...v2.1.0
+[2.0.0]: https://github.com/munich-quantum-toolkit/ddsim/compare/v1.24.0...v2.0.0
 
 <!-- Other links -->