Update devise-two-factor gem to v5

What does this MR do and why?

This MR updates devise-two-factor gem to version 5.1.0.

We are relying on https://github.com/devise-two-factor/devise-two-factor/blob/main/UPGRADING.md#upgrading-from-4x-to-5x, and covering Phase 1.

---

note: In a follow-up MR, we'll address the Phase 2: Clean up

note: After both Phases are fully completed, we'll update the gem to the latest version!

References

• Upgrade devise-two-factor and devise-pbkdf2-enc... (#422927)
• [Cells 1.0] (Size: S) Migrate to `devise-two-fa... (#451147)

Screenshots or screen recordings

N/A

How to set up and validate locally

Via console

1. Generate otp_secret for a user
2. Checkout this branch
3. Verify that legacy_otp_secret method is returning the same value as should be 🎉

E2E Test

1. Enable 2FA for a user using one of available authenticators
2. Logout from the app
3. Checkout this branch and run migrations
4. Login in and when ask provide the new code
5. Verify that everything works 🎉

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.