Set up Python in mergeback workflow

[henrymercer]

We should have been setting up Python anyway, but the workflow's now failing since ubuntu-slim doesn't come with Python.

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/test 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 adds Python setup to the post-release mergeback workflow and updates the Python setup action version in the release-initialise action. The changes ensure Python is available in the workflow environment, which became necessary after ubuntu-slim runners no longer include Python by default.

Key changes:

• Added Python 3.12 setup step to the post-release mergeback workflow
• Updated Python setup action from v5 to v6 in the release-initialise action

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/post-release-mergeback.yml	Added setup-python@v6 step to ensure Python 3.12 is available
.github/actions/release-initialise/action.yml	Updated Python setup action to v6 and quoted version string for consistency