feat(retries): make standard the default retry mode

[Alan4506]

Do not merge before November 2026. This is the "no sooner than November 2026" rollout announced in #3285.

Summary

Makes standard the default retry mode and removes the AWS_NEW_RETRIES_2026 opt-in gate introduced in #3269. The retry behavior itself is unchanged from what the flag enabled.

legacy mode is still available via AWS_RETRY_MODE=legacy, retry_mode = legacy in the shared config file, or 'retries' => ['mode' => 'legacy']. AWS_NEW_RETRIES_2026 is no longer read.

Changes

• Retry/ConfigurationProvider: DEFAULT_MODE is 'standard'; getDefaultMode() no longer checks the flag.
• ClientResolver, S3Client, StsClient, DynamoDbClient, DynamoDbStreamsClient: the OptIn::isEnabled() branches collapse to the new behavior, so standard/adaptive always use Retry\V3\RetryMiddleware.
• Removed (@internal, only reachable with the flag off): Retry/V3/OptIn, RetryMiddlewareV2, Retry/QuotaManager, and their tests.
• Tests: flag toggling removed; opted-in/opted-out pairs collapse to the new behavior; fallback expectations are standard. Added a zero-config SNS test for the new default (3 attempts, 50 ms base).
• Changelog: .changes/nextrelease/feat-retries-default.json.

Testing

• Unit tests: pass.
• make build: no diff.
• PHPStan and phpcs: no new issues.

Related

• feat: new retries implementation #3269 (gated implementation)
• bugfix: DynamoDB retry defaults on the opt-in path #3368 (bugfix on the opt-in path)
• Announcement: Announcement: Updated default retry behavior (2026) #3285 (will be updated after release)

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[stobrien89]

Looks good except for Aws\Retry\Configuration::__construct() still defaults to legacy