Deprecate user-level permissions

### Is your feature request related to a problem? Please describe.

In 2.137.0, we deprecated group-level permissions for several reasons, which are described here: https://docs.flagsmith.com/system-administration/rbac#deprecated-features

For the same reasons, we should also deprecate user-level permissions. This is especially important now since tag-level permissions can only be applied to roles and not users. For example, user-level permissions look like this:


whereas editing role permissions offers more options:


### Describe the solution you'd like.

Deprecate user-level permissions, add warnings in the UI when using them, and make them less prominent than role-level permissions.

### Describe alternatives you've considered

🤷

### Additional context

_No response_