Upgrade Rust to 1.60.0

[soareschen]

Description

Updating Rust to v1.60.0 so that the result on CI matches the result on local machine.

PSA: When updating the code base with this new rust-toolchain.toml file, make sure that you stop Rust Analyzer and the running editor first before pulling the new change. Otherwise you may encounter error message like:

the 'cargo' binary, normally provided by the 'cargo' component, is not applicable to the '1.60.0-x86_64-unknown-linux-gnu' toolchain

To fix that, you have to first quit Rust Analyzer and run the following to re-add the component in Rustup:

rustup component remove cargo
rustup component add cargo

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests: integration (for Hermes) or unit/mock tests (for modules).
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[mzabaluev]

Another PSA: whenever you want to update the MSRV (which this PR does not do), be sure to follow the checklist in the corresponding issue template

[soareschen]

Another PSA: whenever you want to update the MSRV (which this PR does not do), be sure to follow the checklist in the corresponding issue template

Updated the Rust MSRV as well.

[romac]

Let's wait until Monday to merge this to give upstream projects a chance to catch up as well.