Update rustc version (#304)

*Issue #, if available:*
Our recent PR builds for Python are failing with:
```
#16 3.705 error: failed to compile `cargo-audit v0.21.0`, intermediate artifacts can be found at `/tmp/cargo-installzAcK49`.
#16 3.705 To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.
#16 3.705 
#16 3.705 Caused by:
#16 3.705   package `home v0.5.11` cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.75.0
#16 3.705   Try re-running cargo install with `--locked`
#16 ERROR: process "/bin/sh -c if [ $TARGETARCH = \"amd64\" ]; then cargo install cargo-audit && cargo audit ; fi" did not complete successfully: exit code: 101

#15 [build 5/5] RUN mkdir workspace && pip install --target workspace ./aws-opentelemetry-distro
#15 4.601 Collecting opentelemetry-instrumentation-pymemcache==0.48b0 (from aws-opentelemetry-distro==0.7.0.dev0)
#15 4.614   Downloading opentelemetry_instrumentation_pymemcache-0.48b0-py3-none-any.whl.metadata (1.9 kB)
#15 4.657 Collecting opentelemetry-instrumentation-pymongo==0.48b0 (from aws-opentelemetry-distro==0.7.0.dev0)
#15 4.674   Downloading opentelemetry_instrumentation_pymongo-0.48b0-py3-none-any.whl.metadata (1.9 kB)
```

*Description of changes:*
Bumping the version of rustc to `1.81` in our Dockerfile.


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Author: yiyuan-he

Dockerfile

@@ -21,7 +21,7 @@ RUN sed -i "/opentelemetry-exporter-otlp-proto-grpc/d" ./aws-opentelemetry-distr
 RUN mkdir workspace && pip install --target workspace ./aws-opentelemetry-distro
 
 # Stage 2: Build the cp-utility binary
-FROM public.ecr.aws/docker/library/rust:1.75 as builder
+FROM public.ecr.aws/docker/library/rust:1.81 as builder
 
 WORKDIR /usr/src/cp-utility
 COPY ./tools/cp-utility .