Complete packages.timber.io artifact migration and retire obsolete content

[pront]

Goal

Complete the migration away from packages.timber.io for stable Vector release artifacts, then retire obsolete package-hosted content.

Decisions

• Stable Vector release artifacts are published from GitHub Releases. The website and installer migration is tracked in chore(releasing): use GitHub Release assets for stable downloads #26079.
• Do not preserve or migrate nightly artifacts. Stop publishing them and delete vector/nightly/.
• Do not preserve or migrate custom-build artifacts. Stop publishing them and delete vector/custom/.
• Do not preserve or migrate legacy Timber product artifacts. Delete agent/, cli/, and cloudwatch-logs-lambda-function/.
• Helm chart packages already use GitHub Release assets. Keep helm.vector.dev as the repository-index endpoint, but delete the obsolete packages.timber.io/helm/latest and /helm/nightly content.

Plan

1. After chore(releasing): use GitHub Release assets for stable downloads #26079 is released and the one-minor deprecation period has elapsed, remove stable release handling from scripts/release-s3.sh and the publish-s3 job in .github/workflows/publish.yml. GitHub Releases becomes the sole publisher for stable release artifacts.
2. Remove nightly release handling and delete vector/nightly/.
3. Delete the stable S3 objects and compatibility aliases under vector/ (latest, version aliases, and versioned stable artifacts).
4. Delete the legacy agent/, cli/, CloudWatch Lambda, and obsolete Helm prefixes listed above.
5. Remove custom-build release handling and delete vector/custom/.
6. Delete scripts/release-s3.sh and the publish-s3 job once no channel uses them.

Acceptance criteria

• A stable release no longer writes to packages.timber.io.
• No nightly publishing or nightly objects remain.
• No custom-build publishing or custom-build objects remain.
• Obsolete legacy and Helm prefixes are removed.
• The package S3 publisher and its AWS release credentials are no longer used.

Pending decision

Redundancy.