Merge pull request JabRef#10747 from JabRef/downgradejavafx

downgrade javafx to check if it works

build.gradle

@@ -101,7 +101,7 @@ dependencyLocking {
 }
 
 javafx {
-    version = "21"
+    version = "20"
     modules = [ 'javafx.controls', 'javafx.fxml', 'javafx.web', 'javafx.swing' ]
 }