[dependencies] Move off of gulp-cssnano

[joewiz]

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

We have a dependency on gulp-cssnano, but this library was deprecated and archived 3 years ago, and it's source code repository is no longer available.

Describe the solution you'd like

As described at angular/material#11869 (comment):

the new recommendation is to use cssnano with gulp-postcss.

Describe alternatives you've considered

Remove gulp-cssnano entirely?

Additional context

Moving off of gulp-cssnano will also allow us to upgrade to the latest version of is-svg, v4.2.2; otherwise, we're locked into version 2.0.0.

[joewiz]

Another alternative would be to remove gulp entirely and replace it with equivalent maven plugins.

[duncdrum]

we already have a transient dependency on gulp-postcss, I'd recommend sticking with gulp and just switching plugins. Unless we want to start creating our own build tool plz