fix(gboc): regenerate distribution on gboc-experiment-policy and drop stale ottl pin

[dashpole]

Summary

Restores the gboc-experiment-policy components in google-built-opentelemetry-collector/ that were dropped when manifest.yaml and generated_collector/ were overwritten during a branch merge from master (99cddace), fixes collector regeneration after the v0.162.0 upgrade, and enables Presubmit and Proto CI workflows on PRs targeting gboc-experiment-policy.

Changes

1. Remove stale pkg/ottl v0.160.0 replace (specs/google-built-opentelemetry-collector.yaml): The v0.160.0 pin was added when opentelemetry_contrib_version was 0.160.0. Now that opentelemetry_contrib_version is 0.162.0, the pin downgraded pkg/ottl below what v0.162.0 contrib packages require. Removing the replace lets Go module resolution select v0.162.0.
2. Regenerate google-built-opentelemetry-collector (make regen-google-built-otel): Restores nginx, filestats, udplog, googlepolicy, queuebatch, geoip, kafka, the googlecontrolplane extension and provider, and the local module replaces (pkg/googlepolicy, pkg/event, and root module ../..) in manifest.yaml and generated_collector/.
3. Run CI on PRs targeting gboc-experiment-policy (.github/workflows/presubmit.yaml, .github/workflows/proto.yaml): Adds gboc-experiment-policy to on.pull_request.branches so branch-sync PRs run make presubmit and make compare-all.