fix canary msrv to match upstream smithy-rs (#1232)

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## Motivation and Context
<!-- Why is this change required? What problem does it solve? -->
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Canary is failing on MSRV requirements that changed upstream in
`smithy-rs` to 1.81. Update canary accordingly.

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.github/workflows/canary.yaml

@@ -7,7 +7,7 @@ on:
     branches:
       - "canary*"
 env:
-  tool_rust_version: 1.78.0
+  tool_rust_version: 1.81.0
   rust_nightly_version: nightly-2024-03-15
 
 jobs: