Add roave-backward-compatibility-check to CI pipeline

Author: sebastianbergmann

.github/workflows/ci.yml

@@ -37,6 +37,30 @@ jobs:
       - name: "Run vimeo/psalm"
         run: "php7.3 ./tools/psalm --config=.psalm/config.xml --no-progress --shepherd --show-info=false --stats"
 
+  backward-compatibility:
+    name: Backward Compatibility
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Fetch tags
+        run: git fetch --depth=1 origin +refs/tags/*:refs/tags/*
+
+      - name: Install PHP with extensions
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+          coverage: none
+          extensions: intl
+
+      - name: Run roave/backward-compatibility-check
+        run: ./tools/roave-backward-compatibility-check --from=4.0.0
+
   tests:
     name: "Tests"