Update MSRV section in SDK README (#3589)

The original wording still mentions "alpha", so this PR brings it up to
GA times.

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aws/SDK_README.md.hb

@@ -92,7 +92,10 @@ If you are interested in contributing to the SDK, please take a look at [CONTRIB
 
 ## Supported Rust Versions (MSRV)
 
-The SDK currently requires a minimum of Rust {{msrv}}, and is not guaranteed to build on compiler versions earlier than that. While we are still in alpha, we will be keeping the minimum compiler version two releases behind the latest stable release where possible (so if the latest stable were 1.55, we would be on 1.53). However, we are not making any guarantees around this at present. Increases in minimum required Rust version will be called out in the Release Notes for new releases of the SDK.
+The SDK currently requires a minimum of Rust {{msrv}}, and is not guaranteed to build on compiler versions
+earlier than that. We keep the minimum compiler version two releases behind the latest stable release where
+possible (so if the latest stable were 1.55, we would support 1.53). Increases in minimum required Rust
+version will be called out in the Release Notes for new releases of the SDK.
 
 ## Additional Resources