Update GitHub Actions

Changed:
- Add PHP 8.1, 8.2, 8.3, and 8.4 (allow to fail).

.github/workflows/continuous-integration.yml

@@ -4,8 +4,9 @@ on: [push, pull_request]
 
 jobs:
   test:
-    name: "CI"
+    name: "CI (${{ matrix.php-version }}, ${{ matrix.composer-version }}, ${{ matrix.phpdotenv-version }})"
     runs-on: ubuntu-latest
+    continue-on-error: ${{ matrix.experimental }}
 
     strategy:
       matrix:
@@ -15,18 +16,27 @@ jobs:
           - "7.3"
           - "7.4"
           - "8.0"
+          - "8.1"
+          - "8.2"
+          - "8.3"
         composer-version: ["v2"]
         phpdotenv-version: ["^4.1", "^5.2"]
+        experimental: [false]
         include:
           - php-version: "7.1"
             composer-version: "v1"
             phpdotenv-version: "^4.1"
+            experimental: false
           - php-version: "7.1"
             composer-version: "v1"
             phpdotenv-version: "^5.2"
+            experimental: false
           - php-version: "7.4"
             composer-version: "v1"
             phpdotenv-version: "^5.2"
+            experimental: false
+          - php-version: "8.4"
+            experimental: true
 
     steps:
       - name: "Checkout"