Changing default role removes existing users

[parthp2107]

Affected module
UI/Backend
Describe the bug
Changing defaults roles removes existing users

To Reproduce

1. Assign a role to a user
2. Change the assigned role as the default role
3. Make any other role the default role
4. Check the role of the user which was assigned in step 1.

Screen.Recording.2022-03-21.at.4.25.56.PM.mov

Expected behavior
A clear and concise description of what you expected to happen.

Version:

• OS: [e.g. iOS]
• Python version:
• OpenMetadata version: [e.g. 0.8]
• OpenMetadata Ingestion package version: [e.g. openmetadata-ingestion[docker]==XYZ]

Additional context
NA

cc @mithmatt @darth-coder00