Block version 1.91.0 of Rust. (#1571)

### 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:

* [X] Have you followed the guidelines in our Contributing document?
* [X] Have you checked to ensure there aren't other open [Pull
Requests](https://github.com/fastly/cli/pulls) for the same
update/change?

Author: kpfleming

.fastly/config.toml

@@ -18,7 +18,7 @@ toolchain_constraint = ">= 1.21"           # Go toolchain constraint for use wit
 toolchain_constraint_tinygo = ">= 1.18"    # Go toolchain constraint for use with TinyGo.
 
 [language.rust]
-toolchain_constraint = ">= 1.78.0"
+toolchain_constraint = ">= 1.78.0, < 1.91.0" # Rust 1.91.0 produces broken WASM packages which crash when handling requests
 wasm_wasi_target = "wasm32-wasip1"
 
 [wasm-tools]

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Breaking:
 - breaking(tls-custom): correct 'tls-custom activation enable' command parameters to reflect expected input from API ([#1562](https://github.com/fastly/cli/pull/1562))
+- breaking(compute/build): Block version 1.91.0 of Rust as it produces broken WASM packages. ([#1571](https://github.com/fastly/cli/pull/1571))
 
 ### Enhancements:
 - build(dockerfile-rust): add wasm tools to the rust docker container ([#1552](https://github.com/fastly/cli/pull/1552))