Allow only patch-level bumps for `fastrand` in testing (#3612)

## Motivation and Context
Pins major and minor versions for `fastrand` during testing in
`aws-smithy-runtime`

## Description
`aws-smithy-runtime` uses the `fastrand` crate in a waiters' [unit
test](https://github.com/smithy-lang/smithy-rs/blob/eac52eb69c89d78c1844e9e2b0f0c3413031fc58/rust-runtime/aws-smithy-runtime/src/client/waiters/backoff.rs#L137).
Two days ago, `fastrand` of version 2.1.0 got released (previously
2.0.2). According to [this
PR](https://github.com/smol-rs/fastrand/issues/20), a minor version bump
can cause seed-value-breaking changes.

This PR will only allow patch-level bumps for `fastrand` in
`[dev-dependencies]`.

## Testing
Relies on testing in CI (since CI in the main branch currently fails, if
CI passes in this PR we're good).

----

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