Move to Rust 1.83 (backport #211)

[mergify]

This PR updates the pinned Rust version to 1.83, and updates mmtk-core to mmtk/mmtk-core#1253.

---

This is an automatic backport of pull request #211 done by Mergify.

[mergify]

Cherry-pick of 73e9c84 has failed:

On branch mergify/bp/dev/pr-211
Your branch is up to date with 'origin/dev'.

You are currently cherry-picking commit 73e9c84.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   mmtk/rust-toolchain

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both added:      .github/mergify.yml
	both modified:   mmtk/Cargo.lock
	both modified:   mmtk/Cargo.toml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[udesou]

LGTM.