Cleanup continuous integration configuration (#223)

- lock OS version to specific version (ubuntu-20.04)
- use matrix build to test against different PHP versions (8.0 and 7.4)
- update infection/infection to latest version

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   fix-code-style:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v2
@@ -28,15 +28,20 @@ jobs:
           commit_message: Fix code style!
 
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        php-version:
+          - "7.4"
+          - "8.0"
     steps:
       - name: "Checkout"
         uses: "actions/checkout@v2"
 
       - name: "Install PHP with extensions"
         uses: "shivammathur/setup-php@v2"
         with:
-          php-version: "7.4"
+          php-version: ${{ matrix.php-version }}
 
       - name: "Determine composer cache directory"
         id: "determine-composer-cache-directory"

.github/workflows/reviewdog.yml

@@ -5,7 +5,7 @@ on: [pull_request]
 jobs:
   spellchecker:
     name: Spell check
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: reviewdog/action-languagetool@v1

.github/workflows/website.yml

@@ -13,7 +13,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
       - name: Turnstyle

composer.json

@@ -27,7 +27,7 @@
     "require-dev": {
         "ergebnis/composer-normalize": "^2.8",
         "friendsofphp/php-cs-fixer": "^2.16",
-        "infection/infection": "^0.17.5",
+        "infection/infection": "^0.20.1",
         "phpmd/phpmd": "~2.8.0",
         "phpunit/phpunit": "^9",
         "vimeo/psalm": "^4.4.1"