bugfixes (#3078)

* fix phpdoc
* fix invoice export field order
* prevent empty migration warning
* do not trigger export validation on new timesheets
* update license year
* remove trailing comma in function call for php compatibility
* add new composer plugin config
* bump phpunit schema version

composer.json

@@ -107,7 +107,11 @@
         "preferred-install": {
             "*": "dist"
         },
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "composer/package-versions-deprecated": true,
+            "symfony/flex": true
+        }
     },
     "autoload": {
         "psr-4": {