Update changelog for v4.2.1

Author: JonasAlfredsson

docs/changelog.md

@@ -1,5 +1,8 @@
 # Changelog
 
+### 4.2.1
+- Require all certificate files to have a size greater than zero.
+
 ### 4.2.0
 - Add Ionos DNS authenticator plugin
   - PR by [@mzbik][47].

docs/dockerhub_tags.md

@@ -17,7 +17,8 @@ are updated.
 
 | Major | Minor | Patch | Nginx              |
 | ----: | ----: | ----: | :----------------- |
-| 4     | 4.2   | 4.2.0 | 4.2.0-nginx1.23.4  |
+| 4     | 4.2   | 4.2.1 | 4.2.1-nginx1.23.4  |
+|       |       | 4.2.0 | 4.2.0-nginx1.23.4  |
 |       |       |       | 4.2.0-nginx1.23.3  |
 |       | 4.1   | 4.1.0 | 4.1.0-nginx1.23.3  |
 |       | 4.0   | 4.0.0 | 4.0.0-nginx1.23.3  |