Fix encoding of tx rollup batch message content

This MR aims to fix the batch encoding of a message content which was generating an invalid_utf8_string when encoding non utf8 characters into a string. We are now using bytes.

This is a follow-up MR to complete !4572 (merged)

This MR requires to update the regression outputs.

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