Update bb-common schema

General MR

Summary

• Synced the istio, networkPolicies, and routes schema blocks in chart/values.schema.json with bb-common 1.6.0 using bb-common/scripts/schema.sh. The stale routes schema (additionalProperties: false, no defaults key) rejected the routes.defaults values the Big Bang umbrella injects for egress gateway support (big-bang/bigbang#3452)
• Bumped chart version to 7.3.0-bb.2 and regenerated the README

Relevant logs/screenshots

N/A

Linked Issue

Closes: #318 (closed)

Upgrade Notices

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