[pulimsr]

Do not merge before November 2026. This is the "no sooner than November 2026" rollout announced in #3832

Makes Retry Behavior 2.1 the default retry behavior and removes the AWS_NEW_RETRIES_2026 opt-in.

• Clients with no retry configuration use the standard retry strategy, including clients built with ClientConfiguration(true) or a profile.
• AWS_RETRY_MODE=legacy / retry_mode = legacy keeps the previous DefaultRetryStrategy behavior unchanged.
• An HTTP 429 that doesn't match a modeled error is retried as a throttling error.
• DynamoDB and DynamoDB Streams use 4 attempts and a 25 ms base on all default construction paths, including adaptive mode.
  AWS_MAX_ATTEMPTS / max_attempts still take precedence.
• Adds AdaptiveRetryStrategy(long maxAttempts, double transientBackoffBaseSec).

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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