Etherlink/Kernel: use timestamp from block header

Stacked on !17121 (merged).

What

When computing at stage 1 the timestamp of a flushed blueprint in the future of the current block, we read the block header instead of the block.

Why

Because the block header can be much smaller than the block.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR