Etherlink/Kernel: read_next_blueprint_number does not read a block

Stacked on !17120 (merged).

What

This MR changes the definition of read_next_blueprint_number to use the /evm/current_block_header field from !17119 (merged).

Why

To avoid accessing the world state in stage 1 and to read less data from durable storage.

How

The touched function is unfortunately used in a migration step at which we cannot assume that the storage contains a block header. To circumvent this, the previous definition of the function is kept in the migration::legacy module.

• the first commit introduces the module,
• the second commit copies the function in the module,
• the third and last commit modifies the implementation of the function.

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