fix: remove go-multierror dependency

[aaguiarz]

Remove the github.com/hashicorp/go-multierror dependency by replacing multierror.Error pattern with a simple []error slice and errors.Join().

File: internal/authorizationmodel/model.go:210–232

Changes:

• Replace multierror.Error{} with []error slice
• Replace multierror.Append() calls with append()
• Use errors.Join() instead of returning multierror.Error
• This will also drop the indirect dependency github.com/hashicorp/errwrap