Remove preview label from MongoDB online store docs

The MongoDB online store integration is GA. Remove the '(Preview)'
suffix from the page title and the warning hint block.

Signed-off-by: Casey Clements <casey.clements@mongodb.com>

Author: caseyclements

docs/reference/online-stores/mongodb.md

@@ -1,13 +1,9 @@
-# MongoDB online store (Preview)
+# MongoDB online store
 
 ## Description
 
 The [MongoDB](https://www.mongodb.com/) online store provides support for materializing feature values into MongoDB for serving online features.
 
-{% hint style="warning" %}
-The MongoDB online store is currently in **preview**. Some functionality may be unstable, and breaking changes may occur in future releases.
-{% endhint %}
-
 ## Features
 
 * Supports both synchronous and asynchronous operations for high-performance feature retrieval