Buildkite: Skip PR builds for non-legacy branches

[reakaleek]

What

Make the required buildkite/docs-build-pr check exit 0 early (reporting success without building) when a PR targets a branch that no longer carries legacy AsciiDoc docs.

Why

As docs migrate from this legacy AsciiDoc system to docs-builder, PRs targeting already-migrated branches (stack main/9.x, ECK 3.x, etc.) still trigger the required check and run a useless build. We want those to go green instantly. The check can't simply be removed because it's required.

This supersedes #3233, whose single regex ^([0-8])\.[0-9]+$ broke non-stack-versioned doc sets, as reviewers flagged:

• ECE (cloud) uses milestone branch names (ms-119, ms-105…) → matched nothing → would have wrongly skipped every ECE build.
• ECK (cloud-on-k8s) is legacy only on 2.x/1.x/0.x; 3.x lives in docs-builder but matched ^[0-8]\. → would have wrongly built.

How

• New .buildkite/scripts/legacy_branches.pl derives each repo's legacy branch set directly from conf.yaml — reusing the build's own YAML::LoadFile parser and the branch-resolution convention from lib/ES/Book.pm / lib/ES/BranchTracker.pm (LHS branch keys, map_branches, exclude_branches). It maps the GitHub repo name to the conf repo key via the top-level repos: URL map (handles cases like esf → elastic-serverless-forwarder).
• build_pr.sh consults the helper inside the product-repo path (before cloning) and exits 0 when the target branch isn't in the repo's legacy set. Fails open on any helper error or unknown repo, so a real build is never blocked.
• Deriving from conf.yaml means the guard needs no edits as products migrate — no hardcoded version patterns.

Test plan

Verified the helper locally (Perl YAML present):

• legacy_branches.pl elasticsearch → 8.19 … 0.90, no 9.x
• legacy_branches.pl cloud → ms-* milestones (+ master), no main
• legacy_branches.pl cloud-on-k8s → 0.8 … 2.16, no 3.x
• legacy_branches.pl elastic-serverless-forwarder → resolves via esf key
• legacy_branches.pl clients-team → main (proves map_branches)
• unknown repo → exit 2 (fail open)
• shellcheck .buildkite/scripts/build_pr.sh clean for the new guard

Behavioral: elasticsearch+main → skip 0; +8.18 → build. cloud+main → skip 0; +ms-119 → build. cloud-on-k8s+3.0 → skip 0; +2.16 → build.

Notes

The guard runs on the host agent (docs-ubuntu-2204). libyaml-perl is installed in the build Dockerfile (inside the container) but the host image's Perl YAML status is unconfirmed. The fail-open behavior means no regression if it's missing — the skip simply won't take effect until libyaml-perl is added to the agent image (verify with perl -MYAML -e1).

🤖 Generated with Claude Code

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[Mpdreamz]

This is great and love the pl checking conf.yml

I think we can hardcore main and master as fallback even if the yaml dep is missing?