ci: add team Slack mention to version bump pipeline notifications (elastic#273120)

## Summary

- Add team Slack mention to the blocked-state notification so the team
is pinged when the version bump pipeline needs manual approval
- Add team Slack mention to the pass/fail notification so the team is
informed of pipeline completion or failure

## Context

Version bump pipelines are triggered centrally by the
centralized-version-bump pipeline. Previously, only the release engineer
who triggered the pipeline was notified — leaving teams unaware when
their pipeline was waiting for approval or had finished.

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

Author: 3 people

.buildkite/pipelines/version_bump.yml

@@ -2,6 +2,7 @@ notify:
   - slack:
       channels:
         - '#kibana-operations-alerts'
+      message: "<!subteam^S2XQAJ39Q|kibana-operations-team> Version bump to ${NEW_VERSION} — ${BUILDKITE_BUILD_URL}"
     if: (build.branch == 'main' || build.branch =~ /^[0-9]+\.[0-9x]+\$/) &&
       (build.state == 'passed' || build.state == 'failed')
   - slack: