Remove PostgreSQL subchart and require external database (v4.0.0)

[ChrisJBurns]

⚠️ Breaking Change ⚠️

This PR removes the bundled PostgreSQL subchart from the Pact Broker Helm chart, making this a major version bump to v4.0.0. Users must now provide their own PostgreSQL instance.

Background

Due to Bitnami's decision to discontinue free open-source Helm charts and container images, we've removed the PostgreSQL subchart dependency. After evaluation, we found no suitable drop-in replacements that match Bitnami's quality and maintenance standards.

Changes

• Removed PostgreSQL subchart - No longer bundles PostgreSQL with the chart
• Simplified configuration - Renamed externalDatabase to database as it's now the only option
• Updated templates - Removed all conditional logic for PostgreSQL vs external database
• Enhanced CI testing - Added PostgreSQL deployment to Kind cluster for testing
• Matrix testing - Tests against PostgreSQL versions 13, 14, 15, 16, and 17
• Migration guide - Added comprehensive guide for users upgrading

Migration Required

Users upgrading must:

1. Deploy their own PostgreSQL instance (cloud-managed, self-hosted, or via operator)
2. Update their values to use the new database configuration structure
3. Migrate data from the bundled PostgreSQL if previously used

See ./docs/MIGRATION_GUIDE_v4.md for detailed upgrade instructions.

Rationale

• Most production deployments already use managed PostgreSQL services
• Removes maintenance burden and security concerns of bundled database
• Aligns with cloud-native best practices of using external stateful services
• Simplifies the chart and reduces complexity

Fixes #194