Fix the mainnet case of default constants

What, why, and how

The mainnet constant record defined in src/proto_*/lib_parameters/default_parameters.ml is supposed to reflect the value of all parametric constants on Mainnet. It's actually the case except for two constants which represent the level at which the DAL got activated. This MR updates the record to fix this discrepancy.

Manually testing the MR

@luciano.freitas found the bug by implementing a new RPC returning the value of constants_mainnet in !17165 (merged) and comparing the resulting JSON with a mainnet node. I don't know if there could be a simpler way to test this.

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