[IOS SDK] Update change log to reflect features going into public preview

[spetrescu84]

Proposed changes

Updates the change to specify that the following features are going into public preview:

• Email OTP MFA
• Registration of strong authentication method

Type of change

• [ ] Feature work
• [ ] Bug fix
• [x] Documentation
• [ ] Engineering change
• [ ] Test
• [ ] Logging/Telemetry

Risk

• [ ] High – Errors could cause MAJOR regression of many scenarios. (Example: new large features or high level infrastructure changes)
• [ ] Medium – Errors could cause regression of 1 or more scenarios. (Example: somewhat complex bug fixes, small new features)
• [x] Small – No issues are expected. (Example: Very small bug fixes, string changes, or configuration settings changes)

Additional information

[nilo-ms]

I'm not sure these are the changes we're going to add in the next version. The changes mentioned are already in production