Reduce last finalized block level to head~2

This MR aims to set the last finalized block level (block below which we cannot fork any more) to head~2, thanks to Tenderbake. Thus, we update the shell to reflect this constraint.

Based on !11201 (merged)

Manually testing the MR

This MR was tested thanks to a protocol migration test , see the WIP Tezt commit (to be removed when merging) .

One can run the test and check that, after a protocol migration that switches from 5 to 2 cycles to preserve, the node behaves well. In addition to that, the storage is cemented that way.

[12:54:11.264] [node1] Jan 19 13:54:11.264: merging store up to block level 113
[12:54:11.270] client1 exited with code 0.
/tmp/tezt-292426/1/node1/store/chain_NetXdQprcVkpa/cemented
├── 0_1
├── 10_17
├── 18_25
├── 26_33
├── 2_9
├── 34_73   <- migration with a multi-cycle cycle
├── 74_81
├── 82_89
├── 90_97
├── 98_105
…

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