docs(kafka): clarify Kafka topic retention requirements

[shirshanka]

Summary

This PR clarifies Kafka topic retention requirements in the documentation, addressing common questions from users deploying DataHub in environments with Kafka retention restrictions.

Changes

• Mark MetadataGraphEvent_v4 as deprecated (schema removed in 2021, PR fix(model): Remove unused PDL from pre-nocode days #3659)
• Fix duplicate MetadataGraphEvent_v4 entry in kafka-config.md
• Add description for PlatformEvent_v1 topic
• Update DataHubUpgradeHistory_v1 retention guidance:
  • Recommend 7-30 days instead of infinite retention
  • Clarify that infinite retention is not required when DATAHUB_SYSTEM_UPDATE_WAIT_FOR_SYSTEM_UPDATE=false
  • Explain that GMS only reads the last message on startup, not the entire history

Context

This PR addresses questions raised in the Slack community about Kafka topic retention requirements for production deployments. Users deploying DataHub at scale often face platform team restrictions on Kafka retention policies. The documentation previously suggested infinite retention for DataHubUpgradeHistory_v1, which is not actually required in most configurations.

Key Findings:

DataHubUpgradeHistory_v1:

• GMS seeks to the last message on startup (end offset - 1) to verify the upgrade version
• If the topic is empty, GMS starts normally when DATAHUB_SYSTEM_UPDATE_WAIT_FOR_SYSTEM_UPDATE=false (which is the default in many configs)
• 7-30 day retention is sufficient for most deployments

MetadataGraphEvent_v4:

• Introduced in July 2020 (PR metadata-models 72.0.8 -> 80.0.0 #1756)
• Deprecated and schema removed in December 2021 (PR fix(model): Remove unused PDL from pre-nocode days #3659)
• Topic name exists for backward compatibility only
• No active DataHub code produces or consumes from it

Testing

• Documentation changes only, no code changes
• Markdown formatting validated by pre-commit hooks

Related Links

• Slack discussion: https://datahubspace.slack.com/archives/CV2UVAPPG/p1761744794018329
• Original MetadataGraphEvent introduction: PR metadata-models 72.0.8 -> 80.0.0 #1756 (commit 779eaee)
• MetadataGraphEvent deprecation: PR fix(model): Remove unused PDL from pre-nocode days #3659 (commit 8757543)