Allow both teams to release new smithy-rs versions independently (#1618)

This commit modifies the `CODEOWNERS` to allow both
`@awslabs/rust-sdk-owners` and `@awslabs/smithy-rs-server` teams to
release new versions of smithy-rs independently, without requiring peer
approval from the other team.

Syntax and semantics of `CODEOWNERS`: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners