Update rustc version

[yiyuan-he]

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.

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