Update hardening_manifest maintainer from Steven Terhar to Andrew Dunn

Summary

Steven Terhar (@SteveTerhar) is transitioning and Andrew Dunn (@andunn) should be listed as the primary maintainer in the hardening manifest metadata generated by release-helper.sh.

Change

In release-helper.sh, the updateManifest() function hardcodes the maintainer fields via yq eval. Update:

Field	Current	Proposed
.maintainers[0].email	sterhar@gitlab.com	andunn@gitlab.com
.maintainers[0].name	Steven Terhar	Andrew Dunn
.maintainers[0].username	sterhar	andunn
.maintainers[0].cht_member	false	false

This affects all 16 container images produced by the release pipeline.