Update Rust Toolchain to 1.76.0

[roypat]

Some dependencies have recently adopted a MSRV of 1.74.0, which is higher than our current toolchain version (1.73.0). Update to the newest toolchain currently available, to unblock https://github.com/firecracker-microvm/firecracker/pull/4445

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check CONTRIBUTING.md.

PR Checklist

• [ ] If a specific issue led to this PR, this PR closes the issue.
• [ ] The description of changes is clear and encompassing.
• [ ] Any required documentation changes (code and docs) are included in this PR.
• [ ] API changes follow the Runbook for Firecracker API changes.
• [ ] User-facing changes are mentioned in CHANGELOG.md.
• [ ] All added/changed functionality is tested.
• [ ] New TODOs link to an issue.
• [ ] Commits meet contribution quality standards.

---

• [ ] This functionality cannot be added in rust-vmm.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8bb8831) 81.56% compared to head (cd14a0e) 81.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4457      +/-   ##
==========================================
+ Coverage   81.56%   81.63%   +0.06%     
==========================================
  Files         243      243              
  Lines       29542    29800     +258     
==========================================
+ Hits        24095    24326     +231     
- Misses       5447     5474      +27     

Flag	Coverage Δ
4.14-c5n.metal	78.95% <100.00%> (+0.08%)	:arrow_up:
4.14-c7g.metal	?
4.14-m5d.metal	?
4.14-m5n.metal	78.93% <100.00%> (+0.08%)	:arrow_up:
4.14-m6a.metal	78.08% <100.00%> (+0.09%)	:arrow_up:
4.14-m6g.metal	77.03% <100.00%> (+0.07%)	:arrow_up:
4.14-m6i.metal	78.93% <100.00%> (+0.08%)	:arrow_up:
4.14-m7g.metal	77.03% <100.00%> (+0.07%)	:arrow_up:
5.10-c5n.metal	81.60% <100.00%> (+0.07%)	:arrow_up:
5.10-c7g.metal	?
5.10-m5d.metal	?
5.10-m5n.metal	81.59% <100.00%> (+0.07%)	:arrow_up:
5.10-m6a.metal	80.82% <100.00%> (+0.08%)	:arrow_up:
5.10-m6g.metal	79.91% <100.00%> (+0.06%)	:arrow_up:
5.10-m6i.metal	81.58% <100.00%> (+0.07%)	:arrow_up:
5.10-m7g.metal	79.91% <100.00%> (+0.06%)	:arrow_up:
6.1-c5n.metal	81.60% <100.00%> (+0.07%)	:arrow_up:
6.1-c7g.metal	?
6.1-m5d.metal	?
6.1-m5n.metal	81.59% <100.00%> (+0.07%)	:arrow_up:
6.1-m6a.metal	80.82% <100.00%> (+0.08%)	:arrow_up:
6.1-m6g.metal	79.91% <100.00%> (+0.06%)	:arrow_up:
6.1-m6i.metal	81.58% <100.00%> (+0.07%)	:arrow_up:
6.1-m7g.metal	79.91% <100.00%> (+0.06%)	:arrow_up:

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[zulinx86]

Sorry @roypat . My change adding --locked option caused a merge conflict :( Could you please rebase this PR on the latest main?

[roypat]

LGTM! Should we create a pending PR to unpin Kani not to forget to do it?

opened an issue to the kani team: https://github.com/model-checking/kani/issues/3035