Prepare next dev version

Add unreleased entry to changelog.

Signed-off-by: thc202 <[email protected]>

CHANGELOG.md

@@ -3,6 +3,8 @@ All notable changes to this GitHub action will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [Unreleased]
+
 ## [0.12.0] - 2024-04-02
 ### Changed
 - Update dependencies.
@@ -86,6 +88,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 First release to Marketplace.
 
+[Unreleased]: https://github.com/zaproxy/action-baseline/compare/v0.12.0...HEAD
 [0.12.0]: https://github.com/zaproxy/action-baseline/compare/v0.11.0...v0.12.0
 [0.11.0]: https://github.com/zaproxy/action-baseline/compare/v0.10.0...v0.11.0
 [0.10.0]: https://github.com/zaproxy/action-baseline/compare/v0.9.0...v0.10.0