[TASK] Raise PHPStan to level 9 (#1269)

Fixes #1252

phpstan-baseline.neon

@@ -1,2 +1,6 @@
 parameters:
-	ignoreErrors: []
+	ignoreErrors:
+		-
+			message: "#^Method TTN\\\\Tea\\\\Controller\\\\FrontEndEditorController\\:\\:getUidOfLoggedInUser\\(\\) should return int\\<0, max\\> but returns mixed\\.$#"
+			count: 1
+			path: Classes/Controller/FrontEndEditorController.php

phpstan.neon

@@ -10,7 +10,7 @@ parameters:
     # Don't be overly greedy on machines with more CPU's to be a good neighbor especially on CI
     maximumNumberOfProcesses: 5
 
-  level: 8
+  level: 9
 
   paths:
     - Classes