[AI] Add changelog for server template change (#7525)

Co-authored-by: David Motsonashvili <davidmotson@google.com>

Author: davidmotson

firebase-ai/CHANGELOG.md

@@ -4,6 +4,8 @@
 - [changed] Added support for input and output transcription. (#7482)
 - [feature] Added support for sending realtime audio and video in a `LiveSession`.
 - [changed] Removed redundant internal exception types. (#7475)
+- [feature] Added support for server templates via `TemplateGenerativeModel` and 
+  `TemplateImagenModel`. (#7503)
 
 # 17.4.0