Bump pg_textsearch to 1.3.1, remove pg_cron, publish to Docker Hub

[pyramation]

Summary

• Bump PG_TEXTSEARCH_VERSION 1.2.0 → 1.3.1.
• Remove pg_cron entirely (build stage, shared_preload_libraries, README, Makefile test): platform scheduling moved to app_jobs.scheduled_jobs, so no code depends on pg_cron anymore. It was only preloaded for ad-hoc DBA use.
• CI: in the publish-postgres-plus-manifest job, also create multi-arch manifests on Docker Hub (docker.io/constructiveio/postgres-plus) by cross-registry copying the ghcr digests via docker buildx imagetools create. Gated on DOCKERHUB_ENABLED (both DOCKERHUB_USERNAME and DOCKERHUB_TOKEN repo secrets set) — skips gracefully until the secrets exist.

Link to Devin session: https://app.devin.ai/sessions/f54f736b3cb24848bbb20474018bc261
Requested by: @pyramation

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