Use composer v1 in CI

Not all dev dependencies are v2 ready.

maglnet/composer-require-checker has a dependency to
ocramius/package-versions version which requires v1 composer API.
Once a newer version is required, we can use v2.

.github/workflows/ci.yaml

@@ -70,6 +70,9 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-composer-
 
+      - name: Keep composer v1 until all dev dependencies are v2 ready
+        run: sudo composer selfupdate --1
+
       - name: Install dependencies
         run: composer install --prefer-dist --no-progress --no-suggest --no-plugins
 
@@ -139,6 +142,9 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-composer-
 
+      - name: Keep composer v1 until all dev dependencies are v2 ready
+        run: sudo composer selfupdate --1
+
       - name: Install dependencies
         run: composer install --prefer-dist --no-progress --no-suggest