Prepare release v4.0.0 (#259)

## heroku/procfile

### Changed

- Regex based parser has been removed in favor of a `winnow` based parser combinator, the format is now more strict. See [SPEC.md](spec.md) for more details. ([#255](#255))
    - Keys starting with spaces now emit a warning
    - Underscore key characters (`_`) are now converted to hyphens (`-`) and emit a warning
    - Uppercase key characters are now converted to lowercase and emit a warning
    - Invalid keys now error, previously they were ignored

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]
 
+## [4.0.0] - 2025-01-21
+
 ### Changed
 
 - Regex based parser has been removed in favor of a `winnow` based parser combinator, the format is now more strict. See [SPEC.md](spec.md) for more details. ([#255](https://github.com/heroku/buildpacks-procfile/pull/255))
@@ -111,7 +113,8 @@ 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/v3.2.0...HEAD
+[unreleased]: https://github.com/heroku/buildpacks-procfile/compare/v4.0.0...HEAD
+[4.0.0]: https://github.com/heroku/buildpacks-procfile/compare/v3.2.0...v4.0.0
 [3.2.0]: https://github.com/heroku/buildpacks-procfile/compare/v3.1.2...v3.2.0
 [3.1.2]: https://github.com/heroku/buildpacks-procfile/compare/v3.1.1...v3.1.2
 [3.1.1]: https://github.com/heroku/buildpacks-procfile/compare/v3.1.0...v3.1.1

buildpack.toml

@@ -2,7 +2,7 @@ api = "0.10"
 
 [buildpack]
 id = "heroku/procfile"
-version = "3.2.0"
+version = "4.0.0"
 name = "Heroku Procfile"
 homepage = "https://github.com/heroku/buildpacks-procfile"
 description = "Heroku's Procfile buildpack."