docs: update compatibility matrix EG -> V1.8.1 (#2215)

**Description**

Update the compatibility matrix to specify EG v1.8.1+.

Signed-off-by: Aaron Choo <achoo30@bloomberg.net>

Author: aabchoo

site/docs/compatibility.md

@@ -10,7 +10,7 @@ This document provides compatibility information for Envoy AI Gateway releases w
 
 | AI Gateway | Envoy Gateway                 | Kubernetes | Gateway API | Support Status |
 | ---------- | ----------------------------- | ---------- | ----------- | -------------- |
-| main       | v1.7.x+ (Envoy Proxy v1.37.x) | v1.32+     | v1.4.x      | Development    |
+| main       | v1.8.1+ (Envoy Proxy v1.38.1) | v1.32+     | v1.4.x      | Development    |
 | v0.6.x     | v1.7.x+ (Envoy Proxy v1.37.x) | v1.32+     | v1.4.x      | Supported      |
 | v0.5.x     | v1.6.x+ (Envoy Proxy v1.35.x) | v1.32+     | v1.4.x      | Supported      |
 | others     | N/A                           | N/A        | N/A         | End of Life    |