Rollup merge of #123042 - dpaoliello:prelude, r=Nilstrieb

Import the 2021 prelude in the core crate

The `core` crate currently imports the v1 prelude
b3df0d7e5e/library/core/src/lib.rs (L285-L287)

This recently caused an issue when updating the `portable-simd` subtree since it was using a trait that was added to the 2021 prelude: https://github.com/rust-lang/rust/pull/122905#discussion_r1536228822

To make it easier to have a consistent build environment for subtrees and submodules that get included in `core`, we will now import the 2021 prelude into `core`.

Fixes #122912

r? `@Nilstrieb`

library/core/src/lib.rs

@@ -286,7 +286,7 @@ extern crate self as core;
 
 #[prelude_import]
 #[allow(unused)]
-use prelude::v1::*;
+use prelude::rust_2021::*;
 
 #[cfg(not(test))] // See #65860
 #[macro_use]