CI: run linters with PHP 7.4 to work around parsing issues

See https://github.com/vimeo/psalm/issues/4961

.gitlab-ci.yml

@@ -42,7 +42,7 @@ psalm:
   stage: test
   allow_failure: true
   script:
-    - sudo update-alternatives --set php /usr/bin/php7.3
+    - sudo update-alternatives --set php /usr/bin/php7.4
     - composer install
     - composer run psalm
 
@@ -50,7 +50,7 @@ phpstan:
   stage: test
   allow_failure: true
   script:
-    - sudo update-alternatives --set php /usr/bin/php7.3
+    - sudo update-alternatives --set php /usr/bin/php7.4
     - composer install
     - composer run phpstan
 
@@ -58,6 +58,6 @@ cs-fixer:
   stage: test
   allow_failure: true
   script:
-    - sudo update-alternatives --set php /usr/bin/php7.3
+    - sudo update-alternatives --set php /usr/bin/php7.4
     - composer install
     - composer run cs