Improve MixedMode validation
Improve MixedMode validation

- Along with destination Login mode validation, on we validate if login
type is SQLlogin. Windows Login don't need mixedMode enabled.
- Change 'is' to 'was'

dsolodow/dbatools,Splaxi/dbatools,alevyinroc/dbatools,wsmelton/dbatools,sqlpadawan/dbatools,SirCaptainMitch/dbatools,kozloski/dbatools