ci: add team Slack mention to version bump pipeline notifications

[ninalee12]

Summary

Add an @ml-team Slack subteam mention to the version-bump approval notification posted to #machine-learn-build.

The mention is added in .buildkite/pipelines/send_slack_version_bump_notification.sh, which runs after the bump steps and notifies the channel when action may be required (e.g. a GitHub PR needs review/approval).

This covers both workflows:

• Patch (WORKFLOW=patch) — when a version-bump PR is opened
• Minor feature freeze (WORKFLOW=minor) — when the release branch is created and/or a main-bump PR is opened

Context

Version bump pipelines are triggered centrally by release-eng (unified-release-centralized-version-bump). Previously, notifications in #machine-learn-build did not ping @ml-team, so the team could miss when a bump PR was waiting for approval (e.g. ml-cpp-version-bump #58 (https://buildkite.com/elastic/ml-cpp-version-bump/builds/58) / #3068).

This change targets the active notification script used since #3064 (#3064). It does not modify build blocked/finished notifications — the legacy send_version_bump_notification.sh script was removed in that PR.

Feel free to suggest a different Slack handle or a dedicated person to ping if this isn't the right one for your team.

Test plan

• Verify Slack notification in the team channel includes the team mention on next version bump run
• Confirm the mention appears for both patch and minor-freeze notifications when a PR is opened or approval is needed

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[prodsecmachine]

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[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[edsavage]

Hi Nina. The sub-team mention name looks correct, but the change would be better placed in .buildkite/pipelines/send_slack_version_bump_notification.sh -- The send_version_bump_notification.sh script is actually orphaned - I've cleaned it up in #3064 (the PR for the minor version bump automation)

[edsavage]

LGTM