chore: prepare release v3.21.0 (#471)

Author: kangasta

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+## [3.21.0] - 2025-07-15
+
 - Add `zone devices` and `zone devices show` commands for listing available devices.
 
 ## [3.20.2] - 2025-07-09
@@ -594,7 +596,8 @@ Initial public beta release :tada:
 ### Added
 - Current feature set added! First internal release
 
-[Unreleased]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.20.2...HEAD
+[Unreleased]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.21.0...HEAD
+[3.21.0]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.20.2...v3.21.0
 [3.20.2]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.20.1...v3.20.2
 [3.20.1]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.20.0...v3.20.1
 [3.20.0]: https://github.com/UpCloudLtd/upcloud-cli/compare/v3.19.1...v3.20.0