Rollup merge of #79133 - pietroalbini:simplify-stage0, r=Mark-Simulacrum

Rollup merge of #79133 - pietroalbini:simplify-stage0, r=Mark-Simulacrum

bootstrap: use the same version number for rustc and cargo

Historically the stable tarballs were named after the version number ofthe specific tool, instead of the version number of Rust. For example, both of the following tarballs were part of the same release:

    rustc-1.48.0-x86_64-unknown-linux-gnu.tar.xz
    cargo-0.49.0-x86_64-unknown-linux-gnu.tar.xz

PR #77336 changed the dist code to instead use Rust's version number for all the tarballs, regardless of the tool they contain:

    rustc-1.48.0-x86_64-unknown-linux-gnu.tar.xz
    cargo-1.48.0-x86_64-unknown-linux-gnu.tar.xz

Because of that there is no need anymore to have a separate `cargo` field in `src/stage0.txt`, as the Cargo version will always be the same as the rustc version. This PR removes the field, simplifying the code and the maintenance work required while producing releases.

r? ``@Mark-Simulacrum``