Support 2048 leaves tree and remove unneeded forced upgrade

[StanislavBreadless]

What ❔

• The maximal & default size of L2->L1 logs now is 2048.
• The upgrade mechanism that was required for the virtual block migration is no longer needed.

Why ❔

• We can now support any L2->L1 log tree, but due to the server requirements, 2048 will be for now as the constant.
• The upgrade mechanism for the system context is not needed anymore and so can be removed to avoid dead code

Checklist

• [+] PR title corresponds to the body of PR (we generate changelog entries from PRs).
• [+] Tests for the changes have been added / updated.
• [+] Documentation comments have been added / updated.
• [+] Code has been formatted via zk fmt and zk lint.