Update cssnano dependency to include their z-index fix

[michaelfaith]

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• extract-i18n
• run
• config
• help
• version
• doc

Is this a regression?

Yes, the previous version in which this bug was not present was: pre 12.0

Description

Per #20870 cssnano had a bug in 5.0.2, which caused z-indexes to be prefixed by #. They've fixed it (cssnano/cssnano#1122), and released 5.0.4, but /build-angular's package is still locked at 5.0.2.

[angular-automatic-lock-bot]

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