Upgrade the `branch-label` action to use Node.js v20

[eason9487]

Changes proposed in this Pull Request:

Part of #108

This PR upgrades the branch-label action to use Node.js v20.

• The actions-ecosystem/action-add-labels action has not been updated for almost 4 years. Therefore, this PR changes to use an alternative actions/labeler action to achieve the same function.
• In addition, to avoid the issue of not having permission to add labels to pull requests, the example changes the recommended event to the pull_request_target instead of pull_request.

Detailed test instructions:

📌 Using Node.js v20

1. View a previous workflow run used v1 action
   • https://github.com/woocommerce/google-listings-and-ads/actions/runs/8826963398
   • There are warnings of using Node.js 16 or even Node.js 12
2. View test workflow runs used updated action
   • https://github.com/eason9487/grow/actions/runs/8845076148
   • There's no more warning of using Node.js 16

📌 View testing PRs to see if the alternative implementation can achieve the same results

1️⃣ Add a label: https://github.com/eason9487/grow/pull/24

2️⃣ Add multiple labels: https://github.com/eason9487/grow/pull/25

3️⃣ Mismatched branch name: https://github.com/eason9487/grow/pull/26

No labels are added because the branch name fixed/space-time-rift didn't match any rules.

4️⃣ No required permissions: https://github.com/eason9487/grow/pull/29

This test explains that a workflow uses pull_request event may result in permission issue when a PR is created from forks.