jQuery 4 warning: Number-typed values are deprecated for jQuery.fn.css ("z-index", value)

[mgol]

When used against jQuery Git (future jQuery 4), there are warnings when .css() is used against z-index with a number value. This doesn't make sense as z-index never had px auto-appended. d25ecd6 was supposed to solve a similar problem but it only works fine for 3.x, we need to tweak the logic.

Originally reported in jquery/jquery-ui#1960.

[mgol]

PR: #439