Add workflow for rolling back release

[mbg]

This PR adds a new workflow that can be manually triggered to create a rollback release. I.e. the workflow creates a new release which mirrors an older, previous release. Since we publish immutable releases, this is necessary to "revert" a broken release.

The process is:

• We checkout the repository at main. (For testing on a PR only: we checkout the repository at the PR head instead.)
• We create a new branch where we update the changelog for the "new" version to include a message indicating that it is a rollback. This is implemented by a new rollback_changelog.py script. There is some duplication among the Python scripts that could be neater, but I didn't want to tackle big improvements there as part of this PR.
• We then tag the resulting commit with the new release version and update the major version tag.
• We push both tags. (For testing on a PR only: we skip this.)
• We then generate a partial changelog for the new version using the existing prepare_changelog.py script.
• We then create the release as a draft. (Skipped for PR testing.)
• Like in the normal release process, we then create a mergeback PR for the changelog and package.json changes targeting main. (For testing on a PR only: we skip creating the PR, but still push the branch.)

Once the process has completed:

• Since the release is a draft, we have an opportunity to review this to check that we are happy (which is still mutable).
• If we are happy, we can then publish the draft release and then merge the mergeback PR.

This is best reviewed commit-by-commit.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.