Replace postgres jobs with postgres release

[ZPascal]

Summary

This PR removes the postgres and postgres-13 jobs that have been bundled inside the bosh release and replaces them with the cloudfoundry/postgres-release, which ships PostgreSQL 15, 16, 17, and 18 with built-in pg_upgrade support for major-version upgrades.

What changed

• Removed jobs/postgres (fully deleted) and its packages (postgres-13, postgres-15), blob entries, and the CI blob-bumping tasks that maintained them
• Deprecated jobs/postgres-13 — the job spec now carries a deprecation notice directing operators to postgres-release; the job will be removed in the next bosh release
• Added docs/postgres-migration.md with a property-mapping table and in-place cutover procedure for operators switching from either old job
• Updated CI pipeline to test against PostgreSQL 15 and 17 (replacing postgres-13 matrix), expose PG_VERSION env var to the director unit test tasks, and add four new BRATS acceptance jobs (postgres-release 15/16/17/18)
• Added src/brats/acceptance/postgres_release_test.go — matrix BRATS tests that verify the bosh director works correctly against each postgres-release version
• Updated BRATS manifests to use databases.* properties instead of the old postgres.* namespace; postgres_version_helper.rb now reads PG_VERSION from the environment instead of the bosh job spec

Why

The in-tree postgres jobs have been a maintenance burden: every PostgreSQL minor release required a blob bump, a packaging script change, and a new Docker image. postgres-release is a dedicated, well-maintained release that handles packaging, upgrades, and OS compatibility. Offloading PostgreSQL to postgres-release lets us stop duplicating that work and gives operators access to more recent PostgreSQL versions (16, 17, 18) that bosh never shipped.

Migration path for operators

See docs/postgres-migration.md for the full guide. The short version: add postgres-release to your director manifest, replace the postgres.* properties with databases.* properties, and set databases.version to match your existing on-disk PostgreSQL major version. An existing data directory is detected and reused — no dump/restore required.

Test plan

• CI passes: unit-director-postgres-15, unit-director-postgres-17, integration-postgres, integration-postgres-hotswap
• BRATS acceptance jobs pass: brats-postgres-15, brats-postgres-16, brats-postgres-17, brats-postgres-18
• postgres-13 deprecation warning appears in bosh deploy output when the job is still present

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