Add istio-egress-gateway entry to package-mapping.yaml

Summary

istio-egress-gateway package releases skip the auto Big Bang MR because the repo name (istio-egress-gateway) has no entry in tests/package-mapping.yaml, so valuesKey falls back to the repo name and fails to match the istioEgressGateway key in chart/values.yaml.

This adds the mapping entry so map_reponame_to_values_key resolves the repo to istioEgressGateway.

Relevant logs/screenshots

N/A

Linked Issue

Closes #3463 (closed)

Upgrade Notices

N/A