Workflows: Remove cherry-pick label and add failure label on conflict

[Bingtagui404]

What?

Closes #76157

When the automatic cherry-pick workflow fails due to a conflict, remove the Backport to WP X.Y Beta/RC label and add a Backport Failed label.

Why?

Currently, when a cherry-pick fails, no modifications are made to the labels. This makes it hard to notice failures and difficult to retry (re-adding the label triggers a new attempt).

How?

Added a new step in the cherry-pick-wp-release.yml workflow that runs when conflict == 'true'. It mirrors the existing success-path label handling:

• Removes the Backport to WP X.Y Beta/RC label
• Adds the Backport Failed label

This step runs before the conflict comment step, so the label state is updated before the user is notified.

Testing Instructions

1. This change only affects the GitHub Actions workflow file.
2. To test, a cherry-pick conflict would need to occur on a PR with a Backport to WP X.Y Beta/RC label.
3. Verify that after the conflict, the original label is removed and Backport Failed is added.

Use of AI Tools

This PR was authored with the assistance of Claude Code (AI coding assistant). The change was reviewed and validated by the contributor.

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