Block version 1.91.0 of Rust.

[kpfleming]

Change summary

The Fastly engineering team has confirmed that Rust 1.91.0 produces broken WASM packages which will crash when handling incoming requests in Compute services. This breaking change blocks that version when fastly compute build, fastly compute deploy, or fastly compute publish are executed.

Users of this version of the CLI will see a message similar to this:

✓ Verifying fastly.toml
✓ Identifying package name
✓ Identifying toolchain

ERROR: version '1.91.0' of Rust has not been validated for use with Fastly Compute.

Consult the Rust guide for Compute at https://www.fastly.com/documentation/guides/compute/rust/ for more information.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?