ci: drop centos7 from workflow

.github/workflows/ci.yml

@@ -65,36 +65,6 @@ jobs:
           path: |
             test/results/*
 
-  # CentOS 7 Docker: old platform still common in EDA environments.
-  linux-centos:
-    if: github.repository_owner != 'The-OpenROAD-Project-private'
-    runs-on: ubuntu-latest
-    timeout-minutes: 30
-    steps:
-      - name: Checkout
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7.0.0
-        with:
-          submodules: true
-
-      - name: Build image
-        run: docker build -f Dockerfile.centos7 --tag sta-centos7 .
-
-      - name: Test in container
-        run: |
-          mkdir -p test/results
-          docker run --rm \
-            -v "$PWD/test/results:/OpenSTA/test/results" \
-            --entrypoint /bin/bash sta-centos7 \
-            -c "cd /OpenSTA/test && ./regression || (cat results/diffs && exit 1)"
-
-      - name: Upload Test Result
-        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7.0.1
-        if: ${{ !cancelled() }}
-        with:
-          name: result-centos7
-          path: |
-            test/results/*
-
   # Native macOS coverage via Brewfile.
   macos:
     if: github.repository_owner != 'The-OpenROAD-Project-private'