chore: prepare release v5.21.0 (#750)

Author: kangasta

CHANGELOG.md

@@ -5,6 +5,10 @@ See updating [Changelog example here](https://keepachangelog.com/en/1.0.0/)
 
 ## [Unreleased]
 
+## [5.21.0] - 2025-04-15
+
+### Added
+
 - upcloud_server: add `hot_resize` attribute, allowing plan changes without server restarts when supported by the platform. See [UpCloud documentation](https://upcloud.com/docs/guides/scale-cloud-servers-hot-resize/) for more information.
 
 ### Changed
@@ -937,7 +941,8 @@ Updated upcloud-go-api, added build/CI scripts, and repackaged 0.1.0 as 1.0.0.
 - resource_upcloud_firewall_rule removed and replaced by resource_upcloud_firewall_rules
 - resource_upcloud_zone removed and replaced by zone and zones datasources
 
-[Unreleased]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.20.5...HEAD
+[Unreleased]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.21.0...HEAD
+[5.21.0]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.20.5...v5.21.0
 [5.20.5]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.20.4...v5.20.5
 [5.20.4]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.20.3...v5.20.4
 [5.20.3]: https://github.com/UpCloudLtd/terraform-provider-upcloud/compare/v5.20.2...v5.20.3