Open mergeback PR last

[henrymercer]

This reduces the likelihood of publishing a tag but not a release.

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.

Which use cases does this change impact?

Dev only

How did/will you validate this change?

• None - I am not validating these changes.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

How will you know if something goes wrong after this change is released?

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.

[Copilot]

Pull Request Overview

This PR reorders workflow steps to create the mergeback PR after the GitHub release is created, reducing the likelihood of publishing a tag but not a release.

• The "Create mergeback branch and PR" step is moved from before the release creation to after it
• The step execution order now ensures the release is fully published before initiating the mergeback process

[henrymercer]

I also changed this so that we will try to open the mergeback even if the tag exists. This means we can retry generating the mergeback if this workflow fails, even if the tag is already published.