Migrate to Node 20, clean up workflows (#2902)

Tests still appear to fail per #2898. Unfortunately, I need actions/runner#2347 to ignore the test failures properly - I need them to be warnings, not hard errors.

.github/workflows/rollback.yml

@@ -7,24 +7,11 @@ concurrency: prr:deploy
 
 jobs:
   pr:
-
     runs-on: ubuntu-latest
 
-    strategy:
-      matrix:
-        node-version: [16.x]
-
     steps:
-    - uses: actions/checkout@v3
-      with:
-        ref: 'next'
-        fetch-depth: 0
-    - name: Use Node.js ${{ matrix.node-version }}
-      uses: actions/setup-node@v3
-      with:
-        node-version: ${{ matrix.node-version }}
-        cache: 'npm'
-    - run: npm ci
+    - run: git clone --depth=0 https://github.com/MithrilJS/mithril.js.git && cd mithril.js
+    - uses: ./.github/actions/setup
     - run: npm run build
     - run: npx pr-release rollback --verbose --target master --source next --verbose --ignore 'package*' --ignore docs/changelog.md --ignore docs/recent-changes.md
       env: