ci(e2e): reduce chainsaw parallelism from 3 to 2

[jensens]

Summary

• The E2E Chainsaw job on main has been red since the merge of feat: spec.pod.volumes + spec.volumeClaimTemplates for SSD-backed storage (#43) #45 (run 26574487815).
• Two heavy tests (cluster-routing, per-backend-directors) intermittently fail with context deadline exceeded on POSTs to the VinylCache validating webhook.
• The same PR branch had two prior E2E failures before passing, so this looks like flakiness under parallel load, not a code regression.
• The operator log around the failures shows the webhook is still answering for other namespaces (it's slow, not dead) and shows StatefulSet optimistic-concurrency conflicts — both consistent with contention rather than a bug.

Dropping --parallel 3 → --parallel 2 to give the single-pod operator/webhook more headroom on the GitHub runner.

Test plan

• E2E Chainsaw job goes green on this PR
• Total wall-clock time stays within the 45-minute job timeout (was ~8m at parallel=3; expect ~10-12m at parallel=2)

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