Rollup merge of #126662 - alexcrichton:warn-on-wasm32-wasi, r=michaelwoerister

Unconditionally warn on usage of `wasm32-wasi`

This commit is a continuation of the work originally proposed in rust-lang/compiler-team#607 and later amended in
rust-lang/compiler-team#695. The end goal is to rename `wasm32-wasi` to `wasm32-wasip1` to reflect WASI's development and distinguish the preexisting target from the `wasm32-wasip2` target that WASI is now developing. Work for this transition began in #120468 which landed in Rust 1.78 which became stable on 2024-05-02.

This implements the next phase of the transition plan to warn on usage of `wasm32-wasi`. This is intended to help alert users that a removal is pending and all release channels have the replacement available as well. This will reach stable on 2024-09-05. The next stage of the plan is to remove the `wasm32-wasi` target some time in October 2024 which means that the removal will reach stable on 2025-01-09. For reference a full schedule of this transition is listed [here].

Currently this implementation is a simple unconditional warning whenever `rustc --target wasm32-wasi` is invoked. As-implemented there's no way to turn off the warning other than to switch to the `wasm32-wasip1` target.

[here]: rust-lang/rust#120468 (comment)

Author: fmease