EVM/Kernel: block gas limit is not optional

What

Gas limit is not optional

Why and how

It was kept optional because the kernel reads the last block to produce a new one. As the kernel running on ghostnet now always put the gas limit we don't need to support None. The node still decodes as optional, which is fine because if the gas limit is absent it defaults to 0, otherwise it decodes the bytes to u64 little endian.

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