Remove POST /commitments

What and why

Stacked on !12886 (merged).

The DAL node offers two RPCs to post a slot: POST /commitments and POST /slot. The POST /slot RPC is more user-friendly, it automatically pads the slot if it is too short and it not only returns the commitment but also its proof so that the result can directly be used to publish the commitment on the L1.

In this MR, I propose to remove the POST /commitments RPC and I rename POST /slot into POST /slots (plural for consistency with other RPC paths).

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