Release notes should all include all new commits

Currently release only include the `HEAD` commit in their description, ex:

https://github.com/google/xls/releases/tag/v0.0.0-4945-g24be2c494 only includes https://github.com/google/xls/commit/24be2c4943ad9a2365d3b13bb62b3b40f20ed76a commit message:
```
Make visitor for block elaborations.

PiperOrigin-RevId: 624326262
```
It would be nice if instead we included markdown text will all the commit since the last release.

By default the release action we use https://github.com/softprops/action-gh-release?tab=readme-ov-file#-external-release-notes default on GitHub automatically generated release notes:
https://docs.github.com/en/repositories/releasing-projects-on-github/automatically-generated-release-notes

We should look into customizing this first and maybe rely on a changelog generator like https://git-cliff.org/ if it suits our needs better.