chore(portainer): revisit the 3.x LTS migration #327

Portainer 3.0, released in October 2026, is a Kubernetes-first line, and 3.0 itself is a short term support release. The renovate PR that bumped portainer/agent to v3 (#326) was closed rather than merged because 3.0 is not recommended for production, the portainer/portainer-ce image line ends at 2.45, and 3.x ships only as Business Edition. The stack stays on 2.45 LTS in the meantime.

When Portainer 3.3 LTS ships, planned for December 2026, move the server and the agent to 3.x together: swap the server image to the Business Edition image, register the 3 Nodes Free license key while the swarm stays at three nodes or fewer, and record the license issue date for the yearly free renewal. Keep the portainer-data volume so existing data carries over.

2.45 LTS support ends in May 2027 and 2.45 is the last 2.x release, so this should land before then.