Add changelog for 2.4.0

CHANGELOG.md

+## 2.4.0 (2025-05-21)
+
+### Added (2 changes)
+
+- [Support `post_mr_plan_comment` in `full-pipeline` template](https://gitlab.com/components/opentofu/-/commit/7b4ee29322c8486325794c36df9b8d9968ec7207) ([merge request](https://gitlab.com/components/opentofu/-/merge_requests/296))
+- [Implement new `policy-enforcement` template](https://gitlab.com/components/opentofu/-/commit/2943b1deb93f26d33575f6b7e1886460ebe40b4d) ([merge request](https://gitlab.com/components/opentofu/-/merge_requests/295))
+
 ## 2.3.0 (2025-05-20)
 
 ### Added (4 changes)