Update redirecting URLs in reference links of workflow

The "Sync Labels" workflow contains reference links to provide additional information to the users and maintainers.

The targets of some of these links have moved since the time they were added. Although the user could still reach the
intended content via a redirect, it is best not to rely on redirects continuing to work indefinitely. So the URLs are
hereby updated to point directly to the target content.

Author: per1234

.github/workflows/sync-labels.yml

@@ -1,7 +1,7 @@
 # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/sync-labels.md
 name: Sync Labels
 
-# See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
+# See: https://docs.github.com/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows
 on:
   push:
     paths:
@@ -83,7 +83,7 @@ jobs:
     steps:
       - name: Set environment variables
         run: |
-          # See: https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#setting-an-environment-variable
+          # See: https://docs.github.com/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#setting-an-environment-variable
           echo "MERGED_CONFIGURATION_PATH=${{ runner.temp }}/labels.yml" >> "$GITHUB_ENV"
 
       - name: Determine whether to dry run