Use GITHUB_TOKEN for Dependabot metadata fetch and fix dependabot.yml formatting

Author: austenstone

.github/dependabot.yml

@@ -9,3 +9,4 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "daily"
+

.github/workflows/copilot-dependabot-update.yml

@@ -18,7 +18,7 @@ jobs:
         id: metadata
         uses: dependabot/fetch-metadata@v2
         with:
-          github-token: ${{ secrets.PAT }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Generate dependency analysis with Copilot
         uses: austenstone/copilot-cli-actions/.github/actions/copilot@main