Rollup merge of #144928 - cuviper:rustc_thread_pool-msrv, r=lqd

Drop `rust-version` from `rustc_thread_pool`

The current `rust-version = "1.63"` was inherited from rayon, but it
doesn't make sense to limit this in the compiler workspace. Having any
setting at all has effects on tools like `cargo info` that try to infer
the MSRV when the workspace itself doesn't specify it. Since we are the
compiler, our only MSRV is whatever bootstrapping requires.

Author: Kobzol