Improve release note generator to add Ironic upstream references

[tuminoid]

Improve release note generator to add Ironic upstream references.

On the bottom of the Ironic-image release notes, there is section that reads:

Release v30.0.0 is based on ironic version 30.0.0
Ironic Release Notes: https://docs.openstack.org/releasenotes/ironic/unreleased.html#relnotes-30-0-0

Full ironic changelog: [openstack/ironic@stable/2025.1...bugfix/30.0](https://github.com/openstack/ironic/compare/stable/2025.1...bugfix/30.0)

This is currently manually added, but should be added to the release note generator.

Source branches can be obtained from the pinned branch (see pinning PR) or have a placeholder for manual input. Upstream release note link should have a placeholder.

NOTE: metal3-io/project-infra#944 is aiming to move release note generator in project-infra, so this must be considered. The tool should and will have repo specific parts like this, and in case this improvement is implemented here first, it must not get lost when moving.

[elfosardo]

/triage accepted