Bump firecracker version to v1.6.0

[github-actions]

This is an automatically generated Issue to release a new version of
Firecracker on our fork.
There are some manual steps to get new releases of our fork
branch published.
This Issue is to track those manual steps.

Note that for now we are mirroring firecracker's releases of v1.1.x, v1.2.x and v1.3.x versions.

To complete work on this Issue:

• Clone our fork (if you do not have it already).
• Add the upstream as a remote: git add remote up https://github.com/firecracker-microvm/firecracker
• Fetch all branches and tags: git fetch && git fetch --tags
• Check out to the relevant macvtap version branch. feature/macvtap for a new v1.2.x release, feature/macvtapv11x for a new v1.1.x release.
• Update the feature branch. git rebase firecracker-v1.3 for a v1.3.x release git rebase firecracker-v1.2 for a v1.2.x release, etc. etc.
• Resolve any merge conflicts if necessary. (There will be merge conflicts.)
• Add any code if necessary. (This will gradually become more necessary, because the older this feature branch gets, the more the code around it upstream will change.)
• Make sure it builds. (tools/devtool build)
• Push the updated firecracker 'feature/macvtap' (or firecracker/macvtapv12, etc) fork branch.
• Create a new tag on the updated firecracker fork branch: git tag -s v1.6.0-macvtap -m v1.6.0-macvtap.
• Push the tag to the fork: git push <remote> v1.6.0-macvtap (A new release will be created.)
• In this repo, trigger the nightly_e2e workflow
• Once the tests have passed you can close the Issue, if the tests fail, either fix them or unrelease the version

Auto-generated by issue-bot.