De-generics-ize Post, PostText.

Hibernate 6 did not like the old scheme.

Change-Id: I060e077648ab54589d5b7574137c10517b4c4114
diff --git a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
index 64fd6f7..7c9e50f 100644
--- a/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
+++ b/src/main/java/eu/mulk/mulkcms2/benki/newsletter/Newsletter.java
@@ -25,5 +25,5 @@
 
   @OneToMany(mappedBy = "owner", fetch = FetchType.LAZY)
   @OrderBy("date")
-  public Collection<Post<?>> posts;
+  public Collection<Post> posts;
 }