fix(@angular-devkit/build-angular): change css optimizer from clean-css with cssnano

[alan-agius4]

Closes #16123 and closes #13854

[filipesilva]

I think this should target master only though, there's risk of changes in behaviour and there isn't a big need to couple that with 9.0.

[IgorMinar]

if we are deciding between making this change in 9.0 vs 9.1 then let's do it in 9.0. If we think the risk of breaking changes is high, then we should not make this change in 9.x at all and wait for v10.

The reasoning here is that this PR fixes features enabled by v9.0 framework changes so it would be good to get them to 9.0 as a fix, but if the risk of making this change is high then we should not make it in 9.x at all.

@alan-agius4 I'll let you assess which one is right given the guidelines above.

[alan-agius4]

Unless @clydin or @filipesilva feel otherwise, I think we should go ahead and put it in version 9 as opposed to version 10. While there is a risk of breakages, I think it is manageable, worst case we can land a trivial fix to disable the rule that causes the problem.

[alan-agius4]

Patch version: #16647

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