Proto/rollup: switch unparsing mode to optimized_legacy

What

In order to correctly parse the soon to be added Ticket constructor in the Kernel SDK, switch the unparsing mode for rollups to Optimized_legacy.

Why

In a following MR, the unparsing will be added to be able to unparse legacy Pair ticket representation and newly introduced Ticket constructor coming from !11588 (merged)

How

First MR of a train. Next MRs will be:

• Adding the unparsing of D_Ticket !11619 (merged)
• Adding the MichelsonTicket type in Kernel SDK !11588 (merged)
• Put legacy Pair under legacy flag !11620 (merged)
• Switch back from Optimized instead of Optimized_legacy !12048 (merged)

Manually testing the MR

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