Rollup merge of rust-lang#125478 - Urgau:check-cfg-config-bump-stage0, r=Mark-Simulacrum

Bump bootstrap compiler to the latest beta compiler

This PR updates the bootstrap compiler, aka stage0 to the latest beta version, since it contains rust-lang/cargo#13925.

It removes those unconditional Cargo warnings:

```
warning: [...]/rust/library/core/Cargo.toml: unused manifest key: lints.rust.unexpected_cfgs.check-cfg
warning: [...]/rust/library/std/Cargo.toml: unused manifest key: lints.rust.unexpected_cfgs.check-cfg
warning: [...]/rust/library/alloc/Cargo.toml: unused manifest key: lints.rust.unexpected_cfgs.check-cfg
```

for all contributors/users of this repository (including CI).

I don't know if that's something we do, or if it's even advisable, feel free to close.

r? `@Mark-Simulacrum`

Author: matthiaskrgr

alloc/Cargo.toml

@@ -45,7 +45,6 @@ optimize_for_size = ["core/optimize_for_size"]
 level = "warn"
 # x.py uses beta cargo, so `check-cfg` entries do not yet take effect
 # for rust-lang/rust. But for users of `-Zbuild-std` it does.
-# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(no_global_oom_handling)',

core/Cargo.toml

@@ -41,7 +41,6 @@ debug_refcell = []
 level = "warn"
 # x.py uses beta cargo, so `check-cfg` entries do not yet take effect
 # for rust-lang/rust. But for users of `-Zbuild-std` it does.
-# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(no_fp_fmt_parse)',

std/Cargo.toml

@@ -102,7 +102,6 @@ test = true
 level = "warn"
 # x.py uses beta cargo, so `check-cfg` entries do not yet take effect
 # for rust-lang/rust. But for users of `-Zbuild-std` it does.
-# The unused warning is waiting for rust-lang/cargo#13925 to reach beta.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(target_arch, values("xtensa"))',