Add the release note for 1.26.4 (#5317)

site/src/pages/release-notes/1.26.4.mdx

@@ -0,0 +1,24 @@
+---
+date: 2023-12-08
+---
+
+## 🛠️ Bug fixes
+
+- Fixed a bug where a <type://RequestLog> isn't complete when an <type://HttpResponse> is cancelled. #5312
+- [gRPC HTTP/JSON transcoding](https://cloud.google.com/endpoints/docs/grpc/transcoding) service now sends
+  400 Bad Request when the request body is invalid JSON. #5306 #5307
+- You no longer encounter misleading logs stating `An error occurred while pushing a context` when an exception
+  is raised in the callbacks of a context-aware future. #5328
+
+## 🙇 Thank you
+
+<ThankYou usernames={[
+  'ikhoon',
+  'jrhee17',
+  'KarboniteKream',
+  'ks-yim',
+  'mauhiz',
+  'minwoox',
+  'mraasvel',
+  'trustin'
+]} />