Smarter changelog version extraction

[M123-dev]

Hey there, firstly, thanks for publishing an in-house workflow to automate the release. We are planning to move over from a handwritten one, though for versioning we are using release please (https://github.com/google-github-actions/release-please-action) to create the changelog.

The headers are not only the version but also the date and a link to the commit history

## [2.5.1](https://github.com/openfoodfacts/openfoodfacts-dart/compare/v2.5.0...v2.5.1) (2023-05-21)

which results in the error

(error) pubspec version (2.5.1) and changelog (2.5.1 (2023-05-21)) don't agree

Instead of using the whole header name, the version should be extracted

[devoncarew]

Thanks for the issue! Note that this package isn't necessarily intended to be widely used; we're publishing it for our own benefit. I'll update the readme to clarify that.

That said, I don't think we're opposed to making the changelog version parsing logic more robust. The main thing we care about is that the pubspec version == the changelog version.