Upgrade MSRV to 1.68.2 (#2745)

## Motivation and Context
This PR upgrades the MSRV to 1.68.2.

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>
Co-authored-by: 82marbag <69267416+82marbag@users.noreply.github.com>