Replace doc_auto_cfg with doc_cfg to allow docsrs build to succeed

[landonxjames]

Motivation and Context

Our docrs builds started failing recently with the error:

[INFO] [stderr]  Documenting aws-sdk-dynamodb v1.94.0 (/opt/rustwide/workdir)
[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]   --> src/lib.rs:21:29
[INFO] [stderr]    |
[INFO] [stderr] 21 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]    |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]    |
[INFO] [stderr]    = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
[INFO] [stderr]    = note: merged into `doc_cfg`
[INFO] [stderr] 
[INFO] [stderr] error: Compilation failed, aborting rustdoc
[INFO] [stderr] 
[INFO] [stderr] For more information about this error, try `rustc --explain E0557`.
[INFO] [stderr] error: could not document `aws-sdk-dynamodb`

The linked issue shows that doc_auto_cfg has recently been merged into doc_cfg. docsrs builds with the latest nightly toolchain, so the change has impacted our doc builds. This PR makes that change in codegen and in all of our runtime crates.

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.
• For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "aws-sdk-rust" in the applies_to key.

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• AWS SDK (ignoring whitespace)
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• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)
• Server Test Typescript (ignoring whitespace)

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