docs: update compatibility table with Socket.IO v4

There is no breaking change at the protocol level.

Reference: https://socket.io/blog/socket-io-4-release/

Author: darrachequesne

src/site/markdown/installation.md

@@ -3,8 +3,8 @@
 | Client version | Socket.IO server |
 | -------------- | ---------------- |
 | 0.9.x  | 1.x |
-| 1.x    | 2.x |
-| 2.x    | 3.x |
+| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-initialization/#allowEIO3)) |
+| 2.x    | 3.x / 4.x |
 
 ## Installation
 The latest artifact is available on Maven Central.

src/site/markdown/migrating_from_1_x.md

@@ -7,8 +7,8 @@ Here is the compatibility table:
 | Java client version | Socket.IO server |
 | -------------- | ---------------- |
 | 0.9.x  | 1.x |
-| 1.x    | 2.x |
-| 2.x    | 3.x |
+| 1.x    | 2.x (or 3.1.x / 4.x with [`allowEIO3: true`](https://socket.io/docs/v4/server-initialization/#allowEIO3)) |
+| 2.x    | 3.x / 4.x |
 
 **Important note:** due to the backward incompatible changes to the Socket.IO protocol, a 2.x Java client will not be able to reach a 2.x server, and vice-versa