Update website stable version to 2.4.2 (apache#5830)

### Motivation

Apache Pulsar 2.4.2 has been done, but we can see that the stable version of the [website](https://pulsar.apache.org/en/) is 2.4.1. That was done intentionally to get around the website indexing issue. 

Currently, Apache Pulsar's website indexing uses the **Algolia** framework, which takes some time. In Apache Pulsar 2.4.1,  users are complaining the website search is broken, this is the problem.

So in release 2.4.2, we published 2.4.2 documentation but didn't promote it as the current stable. We will make sure the  **Algolia** index works before moving 2.4.2 as the current stable.

In next release, we can think about how to improve the website update process for a release.

### Modifications

- Update website stable version to 2.4.2

site2/website/versions.json

@@ -1,6 +1,6 @@
 [
-  "2.4.1",
   "2.4.2",
+  "2.4.1",
   "2.4.0",
   "2.3.2",
   "2.3.1",