Get rid of bls12-381-legacy

Context

Depends on opam-repository!245 (merged)

bls12-381-legacy has been released to be used in env.v{1,2,3} because encodings are more expensive in bls12-381 than in bls12-381-legacy and would have underestimated the gas cost for the BLS opcodes up to Hangzhou2.

Ithaca2 uses bls12-381, and there won't be any new protocol built on top of env.v3. Therefore, it is fine to replace bls12-381-legacy with bls12-381.

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)
• N/A Provide automatic testing (see the testing guide).
• N/A 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