rust: update the minimum version of Rust Debian Bullseye has just been released, and it carries `rustc 1.48.0`. This actually implies a regression that we can't really do anything about in `rhg`. See https://github.com/rust-lang/rust/issues/88825. In short, closed (or bad) standard file descriptors are reopened silently with no way of telling by the Rust runtime before `main` is executed. This means that closed fds are not forwarded to the subprocess we run in case of fallback. This is a bit sad, but probably not something worth worrying too much about. Differential Revision: https://phab.mercurial-scm.org/D11341