Only build 8.x target branches

Author: reakaleek

.buildkite/pull-requests.org-wide.json

@@ -12,6 +12,7 @@
       "allowed_list": ["github-actions[bot]", "renovate[bot]", "mergify[bot]", "dependabot[bot]", "elastic-renovate-prod[bot]", "elastic-observability-automation[bot]"],
       "build_on_commit": true,
       "build_on_comment": true,
+      "target_branch": ["8.x", "8.19", "8.18", "8.17", "8.16", "8.15", "8.14", "8.13", "8.12", "8.11", "8.10", "8.9", "8.8", "8.7", "8.6", "8.5", "8.4", "8.3", "8.2", "8.1", "8.0"],
       "trigger_comment_regex": "run docs-build ?(?<rebuild_opt>rebuild)? ?(?<warn_opt>warnlinkcheck)? ?(?<skip_opt>skiplinkcheck)?",
       "always_trigger_comment_regex": "buildkite test this ?(?<rebuild_opt>rebuild)? ?(?<warn_opt>warnlinkcheck)? ?(?<skip_opt>skiplinkcheck)?",
       "skip_ci_labels": [