Bump Canary rust version to 1.88 (#1378)

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## Motivation and Context
<!-- Why is this change required? What problem does it solve? -->
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Canaries were failing due to still running on 1.86 after we bumped our
MSRV to 1.88 on the most recent release.

Test canary running at
https://github.com/awslabs/aws-sdk-rust/actions/runs/19078560933

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Author: landonxjames

.github/workflows/canary.yaml

@@ -7,7 +7,7 @@ on:
     branches:
       - "canary*"
 env:
-  tool_rust_version: 1.86.0
+  tool_rust_version: 1.88.0
   rust_nightly_version: nightly-2025-08-06
 
 jobs: