Use PHP 7.4 for tools

Author: sebastianbergmann

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
         uses: "actions/checkout@v2"
 
       - name: "Run friendsofphp/php-cs-fixer"
-        run: "php7.3 ./tools/php-cs-fixer fix --diff-format=udiff --dry-run --show-progress=dots --using-cache=no --verbose"
+        run: "php7.4 ./tools/php-cs-fixer fix --diff-format=udiff --dry-run --show-progress=dots --using-cache=no --verbose"
 
   type-checker:
     name: "Type Checker"
@@ -29,10 +29,10 @@ jobs:
         uses: "actions/checkout@v2"
 
       - name: "Update dependencies with composer"
-        run: "php7.3 ./tools/composer update --no-ansi --no-interaction --no-progress"
+        run: "php7.4 ./tools/composer update --no-ansi --no-interaction --no-progress"
 
       - name: "Run vimeo/psalm"
-        run: "php7.3 ./tools/psalm --config=.psalm/config.xml --no-progress --shepherd --show-info=false --stats"
+        run: "php7.4 ./tools/psalm --config=.psalm/config.xml --no-progress --shepherd --show-info=false --stats"
 
   backward-compatibility:
     name: Backward Compatibility