Witness server (sqlserver2008r2 standard) on Windows 7 required to upgrade 2017 standard

Hi Experts !
I need your kind advise my sql server witness is based on windows 7, I am tried to upgrade but windows 7 does not support the version of sqlserver 2017. I am stuck other principal server and mirror server are based on winserver2008r2 . If I keep sqlserver2008r2 in witness server and upgrade sqlsever to 2017 version on principal server and mirror server did witness server will monitor and auto switches between both serves?
Any suggestion or idea?