Revert Proto: switch to 4h cycle

This MR reverts the 4h cycle changes from !17583 (merged), but keeps 6s block time from !19045 (merged).

Expected changes for the protocol constants:

mainnet	Proto S	Proto T-24h	How to compute?
minimal_block_delay	8	6
max_operations_time_to_live	450	600	= preserve_duration 1h: 3600 / minimal_block_delay
delay_increment_per_round	4	3	= minimal_block_delay / 2
hard_gas_limit_per_block	1_386_666	1_040_000	= old / 8 * 6; = hard_gas_limit_per_operation
blocks_per_cycle	10800	14400	= old * 4 / 3
blocks_per_commitment	84	112	= old * 4 / 3
nonce_revelation_threshold	300	400	= old * 4 / 3
vdf_difficulty	2_400_000_000	2_400_000_000	no change

• changes for smart_rollup_... from the 6s block time MR (!19045 (merged)).

The changes are now almost the same as in !12716 (merged) for a 8-second block time.

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