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
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