rust: release version 0.12.0 (#1281)

### Changelog
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### Docs

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### Description

Bump the Rust code to 0.12.0 to release the linear writer feature.

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<table><tr><th>Before</th><th>After</th></tr><tr><td>

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</td><td>

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rust/Cargo.toml

@@ -7,7 +7,7 @@ categories = [ "science::robotics", "compression" ]
 repository = "https://github.com/foxglove/mcap"
 documentation = "https://docs.rs/mcap"
 readme = "README.md"
-version = "0.11.0"
+version = "0.12.0"
 edition = "2021"
 license = "MIT"