ci: add team Slack mention to version bump pipeline notifications

[ninalee12]

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

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)
• run exhaustive tests : Run the exhaustive tests Buildkite pipeline.

[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-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 8d846f5

Failed CI Steps

• 🥼 x-pack integration
• 🥼 x-pack integration
• 🥼 x-pack integration