Document Rust toolchain upgrade process and goals

[legoktm]

Describe the change

We're responsible for keeping the Rust toolchain up to date, so we should have some rough guidelines and instructions.

I think we should aim to stay 2-3 versions within the latest release, with a 6-week release cycle that would have us updating roughly every 3-5 months (of course we can update faster if there's features we want or specific needs).

Not sure we need to specify the list of files to update since that could change (and hopefully we de-duplicate later on), but for completeness it's:

rust-toolchain.toml
2:channel = "1.69.0"

builder/Dockerfile
45:ENV RUST_VERSION 1.69.0

securedrop/dockerfiles/focal/python3/SlimDockerfile
18:ENV RUST_VERSION 1.69.0

.circleci/config.yml
100:      - image: rust:1.69.0

(maybe we add some magic string like RUST_VERSION everywhere to make it easy to grep for)

Test plan can be based off of freedomofpress/securedrop#6832