ci: declare and enforce MSRV of 1.91.1

[jh-block]

Category: infrastructure
User Impact: Distro packagers (Fedora, Debian, etc.) now have a documented minimum Rust version to target, and the project will no longer silently drift above it.

Problem: A Red Hat engineer packaging goose for Fedora hit a build failure because a transitive dependency had jumped its MSRV past Fedora's packaged rustc. goose had no declared MSRV and no CI enforcement, so any cargo update could silently raise the floor and break downstream packagers without us noticing.

Solution: Declare rust-version = "1.91.1" in the workspace Cargo.toml (this matches the current effective floor, driven by aws-smithy-xml 0.60.15 among others) and propagate it to every member crate. Add a rust-msrv CI job that reads the version out of Cargo.toml and runs cargo check --workspace --locked --all-targets against exactly that toolchain, so MSRV violations fail CI on the offending PR. The version lives in one place; CI extracts it, so future bumps are a one-line change.

File changes

Cargo.toml
Added rust-version = "1.91.1" to [workspace.package] as the single source of truth for the MSRV.

crates/*/Cargo.toml (9 files)
Added rust-version.workspace = true so each member crate inherits the workspace MSRV and reports it in cargo metadata.

.github/workflows/ci.yml
Added a new rust-msrv job that extracts the MSRV from Cargo.toml via sed, installs that exact toolchain, and runs cargo check --workspace --locked --all-targets. Uses a dedicated Swatinem/rust-cache key so it doesn't evict the main build cache.

Reproduction Steps

1. Run `cargo metadata --format-version 1 --no-deps | jq '.packages[] | {name, rust_version}'` and confirm every goose crate reports `"1.91.1"`.
2. Install the MSRV toolchain locally: `rustup toolchain install 1.91.1 --profile minimal`.
3. Run `cargo +1.91.1 check --workspace --locked --all-targets` and confirm it builds cleanly.
4. In CI, observe the new `Check MSRV` job running alongside the existing lint and build jobs.