Backport #4436

[wearyzen]

Changes:

• Add a test to make sure there is no change in behaviour in killing firecracker_pid.
• Update the "Known Limitations" in Jailer and update CHANGELOG to highlight it.

Reason:

• With the change in 4259, the Firecracker process will always have a different pid than the Jailer process.
• In our CI we pass --new-pid-ns along with --daemonize and so firecracker pid can be read from the file firecracker.pid present in the jailers root directory.
  However, if there is a change in behaviour that leads to this file not having the actual firecracker pid then it could cause a regression.

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1d34578) 81.55% compared to head (6dd50ea) 81.53%.

❗ Current head 6dd50ea differs from pull request most recent head 1e44308. Consider uploading reports for the commit 1e44308 to get more accurate results

Additional details and impacted files

@@                 Coverage Diff                  @@
##           firecracker-v1.6    #4440      +/-   ##
====================================================
- Coverage             81.55%   81.53%   -0.03%     
====================================================
  Files                   240      240              
  Lines                 29367    29367              
====================================================
- Hits                  23951    23945       -6     
- Misses                 5416     5422       +6     

Flag	Coverage Δ
4.14-c7g.metal	76.95% <ø> (-0.01%)	⬇️
4.14-m5d.metal	78.85% <ø> (-0.03%)	⬇️
4.14-m6a.metal	77.97% <ø> (ø)
4.14-m6g.metal	76.96% <ø> (ø)
4.14-m6i.metal	78.84% <ø> (ø)
5.10-c7g.metal	79.85% <ø> (ø)
5.10-m5d.metal	81.51% <ø> (-0.03%)	⬇️
5.10-m6a.metal	80.73% <ø> (ø)
5.10-m6g.metal	79.85% <ø> (ø)
5.10-m6i.metal	81.51% <ø> (ø)
6.1-c7g.metal	79.85% <ø> (ø)
6.1-m5d.metal	81.51% <ø> (-0.03%)	⬇️
6.1-m6a.metal	80.73% <ø> (ø)
6.1-m6g.metal	79.85% <ø> (ø)
6.1-m6i.metal	81.51% <ø> (ø)

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