Node: conform to cohttp.6

This simple MR aims to conform to cohttp.6 that might be released soon, as it is a blocker for !8946 (merged). The MR keeps the compatibility with cohttp.5.

Manually testing the MR

• Green CI
• make sure that cohttp.6 + resto (dep) + prometheus (dep) are compiling/running (done here https://gitlab.com/nomadic-labs/tezos/-/commits/remy@vendor-cohttp)

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