Improve migration guide

[mojito317]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [ ] Added tests for code changes or test/build only changes
• [ ] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

There have been some questions regarding migration lately. This PR tries to address them.

Approach

I clarified some of the questionable topics in this change:

• why is use not available anymore in the new SDK
• where users should check the server errors and the authentication errors

I also fixed a sentence where are not transferable to was there twice.

My IDE lately breaks the lines automatically, I have not figured out how I can turn it off yet, but I think this is a great feature, and it helps the reviewers to read the long lines more effectively, so I kept these modifications.

Schema & API Changes

• "No change"

Security and Privacy

• "No change"

Testing

• N/A

Monitoring and Logging

• "No change"

[mojito317]

Thanks for the reviews @emlaver, @vmatyusGitHub, @ricellis! I am merging this!