feat: Certification versions V27 and V28

[schneiderstefan]

Update to the specifications related to the new last_install_timestamp and canister_creation_timestamp leaves in the state tree.

[github-actions]

🤖 Here's your preview: https://yvncx-qyaaa-aaaam-aeiqa-cai.icp0.io

[mraszyk]

It'd be less confusing in my opinion if last_install_timestamp is cleared whenever the canister is uninstalled.

[schneiderstefan]

It'd be less confusing in my opinion if last_install_timestamp is cleared whenever the canister is uninstalled.

But it is. The field is missing if the canister is not installed.

[mraszyk]

The field is missing if the canister is not installed.

{ "last_install_timestamp" : S.last_install_timestamp[canister_id] | if C ≠ EmptyCanister } indeed prunes the field from the certified state tree if the canister is empty. But I'd still set last_install_timestamp[A.canister_id] = (deleted) in the transitions that uninstall code to avoid confusion.

[schneiderstefan]

Oh I see. Sorry I didn't understand the suggestion at first. I added this now. Also, I added that the two fields get deleted when the canister gets deleted, along with the environment variables, which were also missing.

[mraszyk]

I fixed a few more places and the PR looks good to me, but I'll only approve it once the replica code changes is rolled out (to prevent accidental merge).