Bump the min PHP version to PHP 7.4

Supporting newer Symfony versions requires supporting variance rules,
which are not available in PHP 7.1.

Author: stof

.github/workflows/ci.yaml

@@ -9,16 +9,14 @@ on:
 
 jobs:
     run:
-        runs-on: ubuntu-18.04
+        runs-on: ubuntu-latest
         strategy:
             fail-fast: false
             matrix:
                 php:
-                    - '7.1'
-                    - '7.2'
-                    - '7.3'
                     - '7.4'
                     - '8.0'
+                    - '8.1'
                 symfony-versions: [false]
                 include:
                     -   description: 'Symfony 4.*'
@@ -48,5 +46,4 @@ jobs:
 
             - name: Run PHPUnit tests
               run: |
-                export SYMFONY_PHPUNIT_REMOVE_RETURN_TYPEHINT=1
                 ./vendor/bin/simple-phpunit

composer.json

@@ -21,7 +21,7 @@
         }
     ],
     "require": {
-        "php": "^7.1.3 || ^8.0",
+        "php": "^7.4 || ^8.0",
         "ext-dom": "*",
         "ext-json": "*",
         "symfony/config": "^4.4 || ^5.0",