Update changelog entries

Author: ysaito1001

.changelog/retry-behavior-2.1-aws-sdk.md

@@ -9,4 +9,4 @@ breaking: false
 new_feature: true
 bug_fix: false
 ---
-Implement retry behavior 2.1, gated behind `BehaviorVersion::v2026_05_15()`. DynamoDB and DynamoDB Streams use a 25ms base backoff (instead of the general 50ms) and increase default max attempts to 4 (from 3). Long-polling operations (SQS `ReceiveMessage`, SFN `GetActivityTask`, SWF `PollForActivityTask`/`PollForDecisionTask`) backoff even when the retry token bucket is empty.
+Implement retry behavior 2.1, opt-in via `AWS_NEW_RETRIES_2026=true` environment variable. Non-throttling errors now use 50ms base backoff (previously 1,000ms). Transient retry quota cost is 14 tokens (previously 5). DynamoDB and DynamoDB Streams use a 25ms base backoff and increase default max attempts to 4 (from 3). Long-polling operations (SQS `ReceiveMessage`, SFN `GetActivityTask`, SWF `PollForActivityTask`/`PollForDecisionTask`) backoff even when the retry token bucket is empty.