Prepare release v4.2.1 (#270)

## heroku/procfile

### Changed

- Updated libcnb to 0.28.1, which includes tracing improvements/fixes. ([#269](#269))

Co-authored-by: heroku-linguist[bot] <136119646+heroku-linguist[bot]@users.noreply.github.com>

Author: heroku-linguist[bot]

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [4.2.1] - 2025-04-03
+
 ### Changed
 
 - Updated libcnb to 0.28.1, which includes tracing improvements/fixes. ([#269](https://github.com/heroku/buildpacks-procfile/pull/269))
@@ -129,7 +131,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Initial release of Rust procfile buildpack, the old Go buildpack is now archived.
 - Re-write logic of Procfile parsing to match Heroku's behavior, which has different behavior from the Go version (that assumed that a Procfile was YAML syntax).
 
-[unreleased]: https://github.com/heroku/buildpacks-procfile/compare/v4.2.0...HEAD
+[unreleased]: https://github.com/heroku/buildpacks-procfile/compare/v4.2.1...HEAD
+[4.2.1]: https://github.com/heroku/buildpacks-procfile/compare/v4.2.0...v4.2.1
 [4.2.0]: https://github.com/heroku/buildpacks-procfile/compare/v4.1.0...v4.2.0
 [4.1.0]: https://github.com/heroku/buildpacks-procfile/compare/v4.0.0...v4.1.0
 [4.0.0]: https://github.com/heroku/buildpacks-procfile/compare/v3.2.0...v4.0.0

buildpack.toml

@@ -2,7 +2,7 @@ api = "0.10"
 
 [buildpack]
 id = "heroku/procfile"
-version = "4.2.0"
+version = "4.2.1"
 name = "Heroku Procfile"
 homepage = "https://github.com/heroku/buildpacks-procfile"
 description = "Heroku's Procfile buildpack."