Modernize pg_cron 1.6.8 package builds

[serhat andic (serhatandic)]

Summary

• Update pg_cron package metadata and the Debian changelog from 1.6.5 to 1.6.8.citus-1.
• Build the new release for supported PostgreSQL 14, 15, 16, 17, and 18, preserving historical version-matrix entries. PostgreSQL 19 remains excluded while it is in beta.
• Replace retired Debian Buster/Bullseye and Ubuntu Bionic/Focal targets with Debian Bookworm/Trixie and Ubuntu Jammy/Noble/Resolute.
• Update packaging tools from v0.8.30 to v0.8.39, pin Python 3.12 on Ubuntu 24.04, refresh the checkout action, and use the GitHub App authentication pattern already used by other extension workflows. The previous tooling failed while building its old greenlet dependency on Python 3.12.
• Consume ${postgresql:Breaks} in the Debian package template so the new builder's LLVM compatibility constraints are retained rather than ignored.

Publishing scope

• Preserve the existing citusdata/community destination and debian-cron release branch.
• Gate publishing on the exact refs/heads/debian-cron ref and pass the full branch name to the uploader. Builds on this PR branch do not publish packages.

Validation

• Built pg_cron 1.6.8 in WSL/Docker for all five target distributions with packaging output validation enabled: 50 Debian packages, covering PostgreSQL 14-18 and their debug-symbol packages.
• Inspected generated package versions, PostgreSQL-specific shared libraries, and LLVM compatibility constraints.
• Confirmed target-platform support in both the builder and Packagecloud uploader; checked metadata consistency and non-release publishing guards.
• Passed the two targeted tooling tests for platform decoding and PostgreSQL version selection.
• Production signing and Packagecloud uploads were not exercised locally.