Replace once_cell with LazyLock in codegen

[FalkWoldmann]

Motivation and Context

Since Rust 1.80, the once_cell::sync::Lazy functionality is available in the std via sync::LazyLock (see #4050).

Description

I removed the references to the once_cell crate and replaced the usage in the codegen part with LazyLock

Testing

I ran the codegen tests as described in the readme locally (M1 mac), which were successful. I also rg'ed on the generated code and found no usages of once_cell.

Checklist

• 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.

Open questions

• I made this a draft since I'm not quite sure whether this is the correct approach, as I'm not familiar with the code, so I'll be happy about feedback!
• I could also remove the lazy_static dependency or do it in a separate PR, whatever you prefer

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