Add organization_id to vulnerability Elasticsearch document

What does this MR do and why?

Add organization_id to vulnerability Elasticsearch document

Adds an organization_id field to the vulnerabilities index mapping and writes it (gated on the mapping migration, schema version 26_06) from project.namespace.organization_id. A schema-version-based backfill reindexes existing documents so they become queryable by the organization-level security dashboard.

This is the Elasticsearch data-model slice; the by_organization_id query filter and finder routing land in a follow-up.

Related to #605374 (closed)