Cherry-picks updating MSRV to 1.70.0 from main to smithy-rs-release-0.56.x (#2959)

## Motivation and Context
Cherry picks MSRV 1.70.0 upgrade ([original
PR](https://github.com/awslabs/smithy-rs/pull/2948

) merged to `main`)

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Co-authored-by: Zelda Hessler <zhessler@amazon.com>