Java: Add release notes for release 5.21.0 (#2219)

* Add new release notes

* Format

---------

Co-authored-by: Alexander Dümont <[email protected]>

Author: bot-sdk-js

docs-java/release-notes/release-notes-15-to-29.mdx

@@ -1,3 +1,24 @@
+## 5.21.0 - August 01, 2025
+
+[All Release Changes](https://github.com/SAP/cloud-sdk-java/releases/tag/rel%2F5.21.0)
+
+### ✨ New Functionality
+
+- Add `TokenCacheParameters` to `OAuth2Options` to configurate token cache duration, expiration delta and cache size.
+
+### 📈 Improvements
+
+- Relax OAuth2 token cache duration to 1hr to avoid unnecessary token refreshes.
+- Disable refresh tokens when obtaining user tokens from IAS.
+  This acts as a workaround for a limitation of IAS, where obtaining a refresh token invalidates the original token.
+
+### 🐛 Fixed Issues
+
+- OData v2 and OData v4: Fix eager HTTP response evaluation for _Create_, _Update_, and _Delete_ request builders in convenience APIs.
+  Previous change of `5.20.0` may have resulted in the HTTP connection being left open after the request was executed.
+- Generic OData Client: Revert behavior change introduced in `5.20.0` that let to HTTP responses not being consumed by default.
+  If you want to use `disableBufferingHttpResponse()` on `ODataRequestRead` or `ODataRequestReadByKey` please switch to `withoutResponseBuffering()` instead.
+
 ## 5.20.0 - June 20, 2025
 
 [All Release Changes](https://github.com/SAP/cloud-sdk-java/releases/tag/rel%2F5.20.0)