Pin actions/labeler to v4 [skip ci] (#2037)

RAPIDS repos are using the `main` branch of https://github.com/actions/labeler which recently introduced [breaking changes](https://github.com/actions/labeler/releases/tag/v5.0.0).

This PR pins to the latest v4 release of the labeler action until we can evaluate the changes required for v5.

Authors:
   - Ray Douglass (https://github.com/raydouglass)

Approvers:
   - AJ Schmidt (https://github.com/ajschmidt8)

.github/workflows/labeler.yml

@@ -6,6 +6,6 @@ jobs:
  triage:
    runs-on: ubuntu-latest
    steps:
-   - uses: actions/labeler@main
+   - uses: actions/labeler@v4
      with:
        repo-token: "${{ secrets.GITHUB_TOKEN }}"