fix(workflows): invalid labels in bump-latest-cdk8s-plus-library (#…

…1570)

The `bump-latest-cdk8s-plus-library` workflow is [failing](https://github.com/cdk8s-team/cdk8s/actions/runs/6562478240):

```console
[Invalid workflow file: .github/workflows/bump-latest-cdk8s-plus-library.yml#L171](https://github.com/cdk8s-team/cdk8s/actions/runs/6562478240/workflow)
The workflow is not valid. .github/workflows/bump-latest-cdk8s-plus-library.yml (Line: 171, Col: 13): A sequence was not expected
```

Fix label definition according to action [documentation](https://github.com/actions-ecosystem/action-create-issue).

> The labels which are added to the created issue when it's created. Must be separated with line breaks if there're multiple labels.

src/k8s-automation.ts

@@ -250,9 +250,7 @@ export class K8sVersionUpgradeAutomation extends Component {
             github_token: '${{ secrets.PROJEN_GITHUB_TOKEN }}',
             title: 'chore: set default branch of cdk8s-plus repo to latest for v${{ needs.check-latest-k8s-release.outputs.latestVersion }} k8s upgrade',
             body: 'The default branch must be manually set to k8s-${{ needs.check-latest-k8s-release.outputs.latestVersion }}/main',
-            labels: [
-              'priority/p0',
-            ],
+            labels: 'priority/p0',
           },
         },
       ],