Upgrade MSRV to 1.75 (#1129)

**DO NOT MERGE WHILE DAILY RELEASE IS ONGOING.**

The same as smithy-lang/smithy-rs#3553 but done
for the `aws-sdk-rust` repository.

---

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

.github/workflows/canary.yaml

@@ -7,7 +7,7 @@ on:
     branches:
     - 'canary*'
 env:
-  tool_rust_version: 1.74.1
+  tool_rust_version: 1.75.0
   rust_nightly_version: nightly-2024-02-07
 
 jobs: