Use bigger runner to publish images (#4237)
<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Low Risk**
> CI/workflow-only change plus a version/changelog bump; low risk beyond
potential runner availability/cost differences.
>
> **Overview**
> Fixes CD Docker publishing failures by switching the `publish-images`
GitHub Actions job runner from `ubuntu-latest` to `ubuntu-latest-m`,
addressing "no space left on device" issues when pulling multi-arch
images.
>
> Updates release metadata by bumping `unstructured/__version__.py` to
`0.20.1` and adding a matching changelog entry describing the CI runner
change.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
f6408c916ab0f2ab2cfedfdcc06b804909a0f9ff. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
