GH Actions: lint against all supported PHP versions

For now, linting against PHP 8.1 is set to be allowed to fail. This should be turned off once PHP 8.1 has been released in 4/5 weeks time.

Optionally, the `test` workflow could be made dependent on the `lint` workflow, so it won't run unnecessarily.

Author: jrfnl

.github/workflows/continuous-integration.yml

@@ -16,6 +16,14 @@ jobs:
   lint:
     name: "Lint"
     runs-on: "ubuntu-latest"
+
+    strategy:
+      fail-fast: false
+      matrix:
+        php-version: [ '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1' ]
+
+    continue-on-error: ${{ matrix.php-version == '8.1' }}
+
     steps:
       - uses: "actions/checkout@v2"
       - uses: "shivammathur/setup-php@v2"