Proto: generate LB subsidy

Because the LB subsidy depends on maximum rewards per block, which in turn depend on the maximum rewards per minute (currently 80 tez), I think it's better to generate it automatically similarly to other constants that depends on the maximum rewards per minute.

Some regression output needed to be regenerated because the constant changed for non-mainnet networks. (Issue #3398 (closed) was raised as vaguely related to this.)

This MR was previously included in !4554 (merged), but I split it in order to separate concerns.

Context

Manually testing the MR

Checklist

• [n/a] Document the interface of any function added or modified (see the coding guidelines)
• [n/a] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [n/a] 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