Add changelog for enabling retries by default (#4454)

Author: vcjana

.changelog/1769183956.md

@@ -0,0 +1,13 @@
+---
+applies_to:
+- client
+- aws-sdk-rust
+authors:
+- vcjana
+references:
+- smithy-rs#4454
+breaking: true
+new_feature: true
+bug_fix: false
+---
+Enable retries by default for AWS SDK clients starting with `BehaviorVersion::v2026_01_12()`. Previously, retries were disabled by default and required explicit configuration. This change only affects AWS SDK builds and does not impact generic Smithy clients.