Remove legacy code <=0.38

[amaury1093]

Summary

We should remove all legacy code for SDK versions 0.38 (incl.) and before.

Problem Definition

Legacy code makes the codebase bigger, tests slower, lints slower. And they are not maintained.

Proposal

• Remove all code that touches SDK v<=0.38. I can only think of simd migrate commands, but maybe there's something else. We would only offer 0.39 onwards migrations.
  • if users want to do migrations of older versions of the SDK, they can still download an old binary.
• Add documentation on how to perform older migrations.

cc @robert-zaremba @alessio

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[alessio]

ACK from me.

[robert-zaremba]

Fully support that. We had few discussion about that.