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

rust/README.rst

@@ -74,8 +74,8 b' Example usage:'
 Developing Rust
 ===============
 
-The current version of Rust in use is ``1.41.1``, because it's what Debian
-stable has. You can use ``rustup override set 1.41.1`` at the root of the repo
+The current version of Rust in use is ``1.48.0``, because it's what Debian
+stable has. You can use ``rustup override set 1.48.0`` at the root of the repo
 to make it easier on you.
 
 Go to the ``hg-cpython`` folder::

tests/test-basic.t

@@ -55,12 +55,13 b' Writes to stdio succeed and fail appropr'
 On Python 3, stdio may be None:
 
   $ hg debuguiprompt --config ui.interactive=true 0<&-
-   abort: Bad file descriptor
+   abort: Bad file descriptor (no-rhg !)
+   abort: response expected (rhg !)
   [255]
   $ hg version -q 0<&-
   Mercurial Distributed SCM * (glob)
 
-#if py3
+#if py3 no-rhg
   $ hg version -q 1>&-
   abort: Bad file descriptor
   [255]