Define supported API7 EE version range and write release notes

[shreemaan-abhishek]

Part of #22.

• Set 3.9.12 as the validated baseline; decide the minimum supported API7 EE version for GA; document both.
• Note: the control-plane checkout VERSION file showed 3.2-0 - 3.7.x-0 — reconcile this discrepancy.
• Write GA release notes that clearly state supported API7 EE versions and the list of unsupported resources (upstream, consumer-group, service-template, plugin-config).