Proto/michelson: Pair ticket under legacy flag

Context

Fourth part of #6643 (closed), this MR makes legacy Pair constructor only usable when legacy flag is set to true.

Fourth MR of a train.

Closes #6833 (closed)

Previous MRs are:

• Switching unparsing mode for rollups to Optimized_legacy !12030 (merged)
• Adding the unparsing of D_Ticket !11619 (merged)
• Adding the MichelsonTicket type in Kernel SDK !11588 (merged)

Next MR will be:

• Switch back from Optimized to 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