[MSRV] Document policy for when/how MSRV will roll forward

Our current MSRV policy is in the README:
https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/README.md#supported-rust-versions-msrv

> The AWS Lambda Rust Runtime requires a minimum of Rust 1.81.0, and is not guaranteed to build on compiler versions earlier than that.

Presumably someday we will want to roll it forward. Projects often say something like, 'we will support a minimum of the last 6 months of rust versions, and will only roll forward if we need a new feature'. It would be nice to clarify that.

We probably should also explicitly state whether a MSRV bump is considered a breaking semver change or no.

At some point in the future, when we are rolling forward MSRV and guaranteeing support for both new and old for a period of time, we probably also want to add some github actions to test against multiple rust versions.