fix(build): switch to cssnano for CSS optimizations

[crisbeto]

Replaces the deprecated gulp-minify-css with gulp-cssnano.

Fixes #9225.

CC @topherfangio, @devversion, @EladBezalel

Some notes after comparing the output to the one from our previous build:

• The builds take a few seconds longer, but the minified file size is reduced by about 6kb.
• It does shuffle around some styles, but it seems to be playing it pretty safe so I doubt that it'll break anything.
• Apparently the old minifier that we were using was converting instances of outline: none to outline: 0. Now the value is preserved properly, but that'll be a minor difference in the output, compared to before. We could go through and convert the values if needed.

[topherfangio]

@crisbeto Can you include some of the notes you mentioned during our meeting so we have a bit more info? Particularly interested in your comparisons.

Otherwise, LGTM 👍

[crisbeto]

Updated.

[devversion]

LGTM as well.

[jelbourn]

LGTM!