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

--- a/tests/test-basic.t	Wed Oct 20 00:57:02 2021 +0200
+++ b/tests/test-basic.t	Wed Aug 25 17:53:19 2021 +0200
@@ -55,12 +55,13 @@
 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]