ci: add main-branch DRA check for minor version-bump workflow

[ninalee12]

Description

During the recent minor version bump (9.5), each team's version-bump pipeline advanced main to the next dev minor (9.6.0-SNAPSHOT) in addition to cutting the release branch. The version-bump pipelines currently verify only that the release-branch DRA artifacts landed (staging + snapshot for ${BRANCH}.json) — they do not verify that main's DRA snapshot actually published at the new version.

This means a minor bump can report success even if main's DRA pipeline never picked up 9.6.0-SNAPSHOT. The gap was identified during the 9.5 feature freeze. Only ml-cpp and kibana had a main-branch check; this brings the remaining repos up to the same standard.

Tracking: elastic/platform-engineering-productivity#3012

What

Adds a two-step sequence, gated on WORKFLOW=minor:

1. generate-master-dra-check — runs after fetch-dra-artifacts. Computes the next dev minor from NEW_VERSION (e.g. 9.5.0 → 9.6.0) and dynamically uploads a child step via buildkite-agent pipeline upload.
2. fetch-master-dra-artifacts (uploaded at runtime) — polls master.json on GCS with elastic/json-watcher until .version equals ${next_minor}-SNAPSHOT.

Patch and major bumps are unaffected — the if: build.env("WORKFLOW") == "minor" guard means neither step is uploaded.

Notes

• The GCS alias for main's DRA artifacts is master.json (not main.json).
• The next minor is derived from NEW_VERSION, not BRANCH.

[mergify]

This pull request does not have a backport label. Could you fix it @ninalee12? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.