chore: re-enable v5 backwards compatibility e2e CI

[mverzilli]

Summary

• re-enable backwards compatibility e2e coverage for v5 artifacts starting at 5.0.1
• gate PRs targeting v5-next by running compatibility tests after normal ci-full runs on the same EC2 instance
• make v5 nightly compatibility failures blocking, matching RC and stable releases
• reuse successful compatibility commands for the same source inputs and historical artifact version
• remove SKIP_COMPAT_E2E flag to avoid confusion with the new RUN_COMPAT_E2E one.
• increase AWS shutdown time to accommodate compat runs

Caching

Each generated compatibility test command includes the end-to-end build hash and CONTRACT_ARTIFACTS_VERSION, so Redis only reuses a successful result for the same build inputs and historical artifacts. Failed results remain uncached, and relevant uncommitted inputs continue to disable caching.

Compatibility-enabled PRs bypass the whole-run CI success marker because its mode-and-tree key does not represent the historical artifact matrix. This policy is selected through RUN_COMPAT_E2E.