Switch the licenses sync from v2 to v3 + rollout

Flip license_configs to v3 behind a flag once the exporter publishes v3 and the chain verifies end to end.

• Flip + feature-flag guard for rollback (sync_configuration.rb).
• Verify on GDK: sync ingests expressions, packages reference them, read path returns them, dependency list + license scanning render them.
• Staging verification against the staging PDS endpoint
• Confirm offline path before flipping.
• FF rollout issue (mirror [FF] Rollout issue for `cyclonedx_license_expre... (#604791 - closed) • Zamir Martins, Nick Ilieskou • 19.3) + re-sync note for upgraded instances.

Blocked by

• PMDB v3 License Distribution via PDS (gitlab-org#23152 - closed)
• Add `spdx_expression` to `pm_licenses` (#606521 - closed) • Igor Frenkel • 19.4
• V3 `SyncConfiguration` + public-R2 connector (#606522 - closed) • Unassigned • 19.4
• Parse the v3 licenses payload (#606523 - closed) • Igor Frenkel • 19.4
• Persist v3 licenses and expressions (#606524 - closed) • Igor Frenkel • 19.4
• Define precedence between expressions and ident... (#606525 - closed) • Igor Frenkel • 19.4
• Resumable / delta v3 sync (#606526 - closed) • Unassigned • 19.4
• PMDB: Exporter: Export SPDX Expressions and Mig... (#596923 - closed) • Orin Naaman • 19.3.