rust: upgrade supported Rust toolchain version

A few months ago¹, a decision was made to move the Rust toolchain target to
whatever Debian Testing was tracking. I didn't have the bandwidth to act on
it until now.

This is starting to be even more problematic than before, now that edition 2021
is out.

The CI has been updated to track the current Debian testing version, 1.61.0.

[1] https://lists.mercurial-scm.org/pipermail/mercurial-packaging/2022-April/000338.html

mercurial/helptext/rust.txt

@@ -76,8 +76,8 b' instructions on how to install from sour'
 MSRV
 ====
 
-The minimum supported Rust version is currently 1.48.0. The project's policy is
-to follow the version from Debian stable, to make the distributions' job easier.
+The minimum supported Rust version is currently 1.61.0. The project's policy is
+to follow the version from Debian testing, to make the distributions' job easier.
 
 rhg
 ===

rust/README.rst

@@ -77,8 +77,8 b' Example usage:'
 Developing Rust
 ===============
 
-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
+The current version of Rust in use is ``1.61.0``, because it's what Debian
+testing has. You can use ``rustup override set 1.61.0`` at the root of the repo
 to make it easier on you.
 
 Go to the ``hg-cpython`` folder::