Migrate to Harbor: make changes in Operator templating

[adwk67]

See #476 for pre-25.3 changes (pushed are to both Nexus and Harbor, with defaults being set to Harbor).

• chore: set defaults to oci #476

Post 25.3

• post 25.3 changes to workflows (remove Nexus: basically revert this commit)
• don't forget e.g. trino-lb. See chore: update to oci references trino-lb#57 (comment)

[adwk67]

Spike: stackabletech/opa-operator#671

[xeniape]

Both post 25.3 tasks done with #502 and stackabletech/trino-lb#84