add changelog entry for pr 4105

Author: FalkWoldmann

.changelog/1747202677.md

@@ -0,0 +1,10 @@
+---
+applies_to: ["client", "server", "aws-sdk-rust"]
+authors:
+- FalkWoldmann
+references: [smithy-rs#4105]
+breaking: false
+new_feature: false
+bug_fix: false
+---
+Replace once_cell with std equivalents