Remove enable-protocol-upgrade-via-soroban-env-host-prev flag and gate the prev host on vnext

[sisuresh]

Description

Resolves #4357

Checklist

• [ ] Reviewed the contributing document
• [ ] Rebased on top of master (no merge commits)
• [ ] Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• [ ] Compiles
• [ ] Ran all tests
• [ ] If change impacts performance, include supporting evidence per the performance document