Bump MSRV to 1.81.0 (#3936)

## Motivation and Context
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## Description
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## Testing
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## Checklist
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- [ ] 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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Co-authored-by: Fahad Zubair <fahadzubair@gmail.com>
Co-authored-by: Fahad Zubair <fahadzub@amazon.com>
Co-authored-by: AWS SDK Rust Bot <97246200+aws-sdk-rust-ci@users.noreply.github.com>
Co-authored-by: ysaito1001 <awsaito@amazon.com>
Co-authored-by: Ben Schofield <47790940+Benjscho@users.noreply.github.com>