Delete the 3.x lab shape once the last installation runs the 4.x meta chart

[teemow]

Problem

agentlab still renders a second shape of the platform values for a released 3.x meta chart (config.LegacyChart: an exact platform.chartVersion below 4.0.0 on the stable channel). The switch exists for the migration rehearsal (#143): a 3.x lab upgraded in place to the 4.x line, which is the recipe every installation's kagent API v2 cut-over follows (epic https://github.com/giantswarm/giantswarm/issues/37705).

The shape is live code with its own tests and documentation: the LegacyChart branches in agent-platform-values.yaml.tmpl and render.go, TestPlatformValuesLegacyChartShape and TestGitHubTokenLegacyChartTakesNoKeys, hack/migration-rehearsal/, docs/migration-rehearsal.md and the 3.x paragraphs in docs/platform.md, docs/models.md and docs/getting-started.md. Every change to the 4.x values has to keep the 3.x render correct too (#205 / #207 was one such cost), for a chart line the lab is not verified with and that no installation will run once the fleet has cut over.

The line cannot go yet: the fleet base still bounds the meta chart to >=2.5.5 <4.0.0 (giantswarm/management-cluster-bases#739), and only the three Giant Swarm installations patch their OCIRepository to the 4.x range. The customer management clusters run the 3.x chart until their cut-over tasks under the epic close; the rehearsal is what those cut-overs rehearse.

Proposed solution

When the last installation task under https://github.com/giantswarm/giantswarm/issues/37705 is closed and the base bound has advanced past 4.0.0 (giantswarm/management-cluster-bases#738), remove the 3.x shape in one PR:

• config.LegacyChart and ChartMajor (if nothing else needs it), the LegacyChart field of the render data and every {{ if .LegacyChart }} branch in the templates;
• gitHubTokenWired collapses to gitHubTokenSet;
• TestPlatformValuesLegacyChartShape, TestGitHubTokenLegacyChartTakesNoKeys, TestLegacyChart and the legacyChartVersion fixture;
• hack/migration-rehearsal/ and docs/migration-rehearsal.md; the 3.x paragraphs in the other docs; the README row.
• agentlab configure / platform refuse a chartVersion below 4.0.0 with a message naming the last agentlab release that supported the rehearsal, so a stale agentlab.yaml fails clearly instead of rendering the 4.x shape against a 3.x chart's closed schema.

Until then this issue is the reminder that the shape is deliberate and gated, not forgotten.

Acceptance criteria

• Every installation task under https://github.com/giantswarm/giantswarm/issues/37705 is closed and extras/agent-platform/oci-repository.yaml in the fleet base bounds the chart at or above 4.0.0.
• No LegacyChart reference remains in internal/, the templates, hack/ or docs/; go test ./... green.
• A platform.chartVersion below 4.0.0 is refused at configure and platform with a message pointing at the last release that supported it.
• The release notes of the agentlab release that drops the shape say so.