chore: drop support for Skylake instances

[Manciukic]

Changes

We're dropping official support for our oldest supported instance type. There is no functional change, meaning Firecracker will continue to work on that instance type, but we will drop all automated testing on it.

Reason

Remove unused instance type.

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

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[Manciukic]

Testing on CPU Templates pipeline: https://buildkite.com/firecracker/cpu-templates/builds/967/steps/canvas

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.17%. Comparing base (f101299) to head (29c7949).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5411      +/-   ##
==========================================
+ Coverage   83.13%   83.17%   +0.04%     
==========================================
  Files         266      266              
  Lines       30533    30533              
==========================================
+ Hits        25383    25396      +13     
+ Misses       5150     5137      -13     

Flag	Coverage Δ
5.10-c5n.metal	?
5.10-m5n.metal	83.23% <ø> (+<0.01%)	⬆️
5.10-m6a.metal	82.54% <ø> (+<0.01%)	⬆️
5.10-m6g.metal	79.87% <ø> (ø)
5.10-m6i.metal	83.23% <ø> (ø)
5.10-m7a.metal-48xl	82.53% <ø> (?)
5.10-m7g.metal	79.87% <ø> (ø)
5.10-m7i.metal-24xl	83.19% <ø> (?)
5.10-m7i.metal-48xl	83.20% <ø> (?)
5.10-m8g.metal-24xl	79.87% <ø> (?)
5.10-m8g.metal-48xl	79.87% <ø> (?)
6.1-c5n.metal	?
6.1-m5n.metal	83.27% <ø> (-0.01%)	⬇️
6.1-m6a.metal	82.58% <ø> (ø)
6.1-m6g.metal	79.87% <ø> (ø)
6.1-m6i.metal	83.27% <ø> (+<0.01%)	⬆️
6.1-m7a.metal-48xl	82.57% <ø> (?)
6.1-m7g.metal	79.87% <ø> (ø)
6.1-m7i.metal-24xl	83.29% <ø> (?)
6.1-m7i.metal-48xl	83.29% <ø> (?)
6.1-m8g.metal-24xl	79.87% <ø> (?)
6.1-m8g.metal-48xl	79.87% <ø> (?)

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[Manciukic]

I fixed the last thing taka mentioned to make the comment clearer

    # Thus, since the FLUSH_L1D bit is masked off prior to kernel v6.4, guests with
    # IA32_ARCH_CAPABILITIES.FB_CLEAR (bit 17) = 0 (like guests with T2S template which presents
    # an Intel Skylake CPU) fall into the MMIO_MITIGATION_UCODE_NEEDED branch, marking the
    # system as vulnerable to MMIO Stale Data.