Migrate sync-labels workflow from deprecated set-output command

GitHub Actions provides the capability for workflow authors to use the capabilities of the GitHub Actions ToolKit
package directly in the `run` keys of workflows via "workflow commands". One such command is `set-output`, which allows
data to be passed out of a workflow step as an output.

It has been determined that this command has potential to be a security risk in some applications. For this reason,
GitHub has deprecated the command and a warning of this is shown in the workflow run summary page of any workflow using
it:

The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more
information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

The identical capability is now provided in a safer form via the GitHub Actions "environment files" system. Migrating
the use of the deprecated workflow commands to use the `GITHUB_OUTPUT` environment file instead fixes any potential
vulnerabilities in the workflows, resolves the warnings, and avoids the eventual complete breakage of the workflows that
would result from GitHub's planned removal of the `set-output` workflow command 2023-05-31.

Author: per1234

.github/workflows/sync-labels.yml

@@ -102,7 +102,7 @@ jobs:
         run: |
           # Use of this flag in the github-label-sync command will cause it to only check the validity of the
           # configuration.
-          echo "::set-output name=flag::--dry-run"
+          echo "flag=--dry-run" >> $GITHUB_OUTPUT
 
       - name: Checkout repository
         uses: actions/checkout@v3