Upgrade MSRV to 1.78.0

[ysaito1001]

Motivation and Context

Upgrades MSRV to 1.78.0 since the latest Rust at the time of writing is 1.80.1 (we try keeping two releases behind).

Description

In Rust 1.78.0, clippy enforces stricter checks in two areas:

• dead_code
  It seems to be able to uncover more unused code patterns, especially when different combinations of cargo features are enabled
• empty_docs
  Prior to 1.78.0, the codegen used to render the following empty docs ///, regardless whether the codegen is for the server or for the client. This is now flagged as empty_docs by clippy:

///
pub(crate) mod XXX {
    ...
}

Other than bumping MSRV to 1.78.0, the remaining code changes address issues related to the fixes mentioned above.

EDIT:
cargo-semver-checks required an upgrade due to incompatibility issues following the MSRV upgrade (example error). Specifically, after updating the MSRV to 1.78.0, the rustdoc version was upgraded to v28. However, the version of cargo-semver-checks we were using (0.24.1) only supported rustdoc up to v27.

To resolve this, we have updated cargo-semver-checks to the first version that supports rustdoc v28.

Testing

Existing tests in CI

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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[github-actions]

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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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[github-actions]

A new generated diff is ready to view.

• 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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[david-perez]

Aren't we affected by https://blog.rust-lang.org/2024/05/06/check-cfg.html when using the aws_sdk_unstable flag?

[ysaito1001]

Aren't we affected by https://blog.rust-lang.org/2024/05/06/check-cfg.html when using the aws_sdk_unstable flag?

The provided link refers to Rust 1.80? Since this PR upgrades to Rust 1.78, we're not there yet (if I understand your question correctly)? If you mean "will we eventually be affected", then the answer is possibly.