Added GH action to close PRs

.github/CONTRIBUTING.md

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+# Discord Rich Presence for Plex
+
+## Contributing
+
+This repository does not accept pull requests.

.github/workflows/close-pull-requests.yml

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+name: Close Pull Requests
+on:
+  pull_request_target:
+jobs:
+  close-prs:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+      contents: write
+    steps:
+    - env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        gh pr comment --repo=${{ github.repository }} ${{ github.event.number }} --body "This repository does not accept pull requests."
+        gh pr close --repo=${{ github.repository }} ${{ github.event.number }}