[backport-kubernetes-1.39] Sync CI configuration with main branch

[mrodm]

TL;DR

Syncs .buildkite and .github CI configuration from main to the
backport-kubernetes-1.39 backport branch.

This brings automated backport workflows, new CI scripts, Go tooling, and
GitHub Actions that only work when they are present on the target branch itself.

Proposed commit message

Sync CI configuration with main branch for backport-kubernetes-1.39.

Keep CI pipelines, GitHub Actions, and tooling consistent between active backport
branches and `main`, so that automated backport workflows and tooling improvements
land on all supported branches.

WHY:
GitHub Actions resolves workflow files from the branch where the event occurs,
not from `main`. Backport-branch workflows (changelog sync, post-backport
checklist, owner checks, auto-backport) must be present on the backport branch
itself to fire correctly.

Author's Checklist

• CI configuration files have been synced verbatim from main.

Related issues

• Relates backport: deploy backport-branch automation workflows to active backport branches #19262

---

This PR was generated with the assistance of Claude (claude-sonnet-4-6).

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

Package crowdstrike 👍(0) 💚(1) 💔(1)

Expand to view

Data stream	Previous EPS	New EPS	Diff (%)	Result
fdr	1776.83	315.16	-1461.67 (-82.26%)	💔

To see the full report comment with /test benchmark fullreport

[infra-vault-gh-plugin-prod]

⏳ Build in-progress, with failures

• Buildkite Build
• Commit: c3accea

Failed CI Steps

• Check integrations activemq
• Check integrations apache
• Check integrations aws
• Check integrations etcd
• Check integrations fortinet_fortimanager
• Check integrations hadoop
• Check integrations haproxy
• Check integrations iptables
• Check integrations jolokia
• Check integrations journald
• Check integrations logstash
• Check integrations nats
• Check integrations nginx
• Check integrations php_fpm
• Check integrations sophos_central
• Check integrations system
• Check integrations websphere_application_server

History

• 💔 Build #47283 failed af4ba01
• 💔 Build #47279 failed 7fd66ce
• 💔 Build #47275 failed e9a6a08
• 💔 Build #47271 failed 6838919

cc @mrodm