THRIFT-5819: use latest rustc version for rustlib

[cameron-martin]

Client: rust

This upgrades the version of rust in the rust-toolchain file, docs and dockerfiles. Doing so requires a few changes to the source, mainly to fix or silence new warnings.

Submitted on behalf of a third-party: Jiayu Liu
Derived from the following PR: #3045

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[cameron-martin]

For legal/licensing reference:

The Apache License Version 2.0 clause 5 implies that by submitting the Contribution (in the original PR), the code from the original author is under the terms of the Apache License Version 2.0.