[TASK] Raise PHPStan to level 6 (#1258)

Part of #1252

phpstan-baseline.neon

@@ -1,2 +1,6 @@
 parameters:
-	ignoreErrors: []
+	ignoreErrors:
+		-
+			message: "#^Method TTN\\\\Tea\\\\Domain\\\\Repository\\\\TeaRepository\\:\\:findByOwnerUid\\(\\) return type with generic interface TYPO3\\\\CMS\\\\Extbase\\\\Persistence\\\\QueryResultInterface does not specify its types\\: ModelType$#"
+			count: 1
+			path: Classes/Domain/Repository/TeaRepository.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: 4
+  level: 6
 
   paths:
     - Classes