ci: add aarch64 to cross-kernel tests

[pb8o]

Also test 6.1 -> 5.10

Changes

Add more tests since they work and we want to validate this functionality forwards and backwards.

Reason

Gain more confidence on the cross-kernel snapshot-restore.

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

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

---

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

[codecov[bot]]

Codecov Report

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

Project coverage is 81.63%. Comparing base (6d43ad7) to head (3e92e07).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4450   +/-   ##
=======================================
  Coverage   81.63%   81.63%           
=======================================
  Files         243      243           
  Lines       29794    29794           
=======================================
  Hits        24322    24322           
  Misses       5472     5472           

Flag	Coverage Δ
4.14-c5n.metal	78.94% <ø> (ø)
4.14-c7g.metal	?
4.14-m5d.metal	?
4.14-m5n.metal	78.93% <ø> (ø)
4.14-m6a.metal	78.07% <ø> (ø)
4.14-m6g.metal	77.03% <ø> (ø)
4.14-m6i.metal	78.93% <ø> (ø)
4.14-m7g.metal	77.03% <ø> (ø)
5.10-c5n.metal	81.60% <ø> (ø)
5.10-c7g.metal	?
5.10-m5d.metal	?
5.10-m5n.metal	81.59% <ø> (ø)
5.10-m6a.metal	80.83% <ø> (ø)
5.10-m6g.metal	79.92% <ø> (ø)
5.10-m6i.metal	81.59% <ø> (ø)
5.10-m7g.metal	79.92% <ø> (ø)
6.1-c5n.metal	81.60% <ø> (ø)
6.1-c7g.metal	?
6.1-m5d.metal	?
6.1-m5n.metal	81.59% <ø> (ø)
6.1-m6a.metal	80.83% <ø> (ø)
6.1-m6g.metal	79.92% <ø> (ø)
6.1-m6i.metal	81.59% <ø> (ø)
6.1-m7g.metal	79.92% <ø> (ø)

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[roypat]

Mh, should we also update https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/snapshot-support.md#snapshot-compatibility-across-kernel-versions?