docs: update plugin migration status in v5 migration plan

[pyramation]

Summary

Updates the Graphile v5 migration plan document with a comprehensive inventory of all plugins from the main branch and their current migration status on develop-v5. This replaces the previous high-level plugin sections with detailed tracking tables.

Key additions:

• Plugins inlined into graphile-settings: 9 presets (InflektPreset, MetaSchemaPreset, TsvectorCodecPreset, etc.)
• Plugins using community packages: connection-filter and many-to-many now use official v5 community packages
• Standalone packages upgraded: 6 packages including graphile-authz (66 tests) and postgraphile-plugin-pgvector (9 tests)
• Packages deleted: graphile-simple-inflector and graphile-plugin-connection-filter
• Packages requiring v5 port: 8 packages identified with priority levels (graphile-pg-type-mappings is high priority)

Review & Testing Checklist for Human

• Verify the plugin inventory accurately reflects the current state of graphile/ directory on develop-v5
• Confirm the priority order for remaining plugins makes sense for your roadmap (pg-type-mappings marked as high priority to inline into settings)

Notes

This is a documentation-only change to help track migration progress. The next recommended action based on this inventory is to inline graphile-pg-type-mappings into graphile-settings.

Link to Devin run: https://app.devin.ai/sessions/ba45b64c92bb4307bf9e920123213d9e
Requested by: Dan Lynch (@pyramation)

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