Repair wp/7.0 branch Actions: pin runner images instead of *-latest

[desrosj]

Part of #82782.

What?

Replaces the floating ubuntu-latest, windows-latest, and macos-latest GitHub Actions runner labels in this branch's workflows with pinned versions: ubuntu-24.04, windows-2025, and macos-15 respectively.

Why?

GitHub has been emitting deprecation warnings for the floating -latest runner image aliases. Upstream trunk made this same change in PR #71201 (commit 82768d0e018b44007e3e3a25636c13375b33e7e7), "Avoid using *-latest for runner images", merged 2025-08-28. This PR ports the same substitution to wp/7.0.

Use of AI Tools

This PR was authored with the help of Claude Code.

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🟢 Approval recommended

The runner substitution is valid, complete, and consistent with the upstream change.

Pull request overview

Pins the Slack notification workflow to a stable Ubuntu runner image.

Changes:

• Replaces ubuntu-latest with ubuntu-24.04.

File summaries

File	Description
.github/workflows/notify-slack-on-label.yml	Pins the notification job’s runner image.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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Co-authored-by: desrosj <desrosj@git.wordpress.org>

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