Update postcss infrastructure.

[kuasha420]

Summary

Addresses issue:

• Modernize Site Kit PostCSS configuration for modern browsers #8414

Relevant technical choices

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Build files for e5c20b4 are ready:

• Download development build
• Download production build

[github-actions]

Size Change: -1.06 kB (0%)

Total Size: 1.43 MB

Filename	Size	Change
./dist/assets/css/googlesitekit-admin-css-********************.min.css	53 kB	-733 B (-1%)
./dist/assets/css/googlesitekit-adminbar-css-********************.min.css	11 kB	-207 B (-2%)
./dist/assets/css/googlesitekit-wp-dashboard-css-********************.min.css	7.36 kB	-120 B (-2%)

ℹ️ View Unchanged

Filename	Size
./dist/assets/css/googlesitekit-authorize-application-css-********************.min.css	731 B
./dist/assets/js/29-********************.js	2.8 kB
./dist/assets/js/30-********************.js	2.28 kB
./dist/assets/js/31-********************.js	3.72 kB
./dist/assets/js/32-********************.js	929 B
./dist/assets/js/33-********************.js	888 B
./dist/assets/js/34-********************.js	3.12 kB
./dist/assets/js/analytics-advanced-tracking-********************.js	778 B
./dist/assets/js/googlesitekit-activation-********************.js	23.8 kB
./dist/assets/js/googlesitekit-ad-blocking-recovery-********************.js	57.4 kB
./dist/assets/js/googlesitekit-adminbar-********************.js	34.2 kB
./dist/assets/js/googlesitekit-api-********************.js	10.1 kB
./dist/assets/js/googlesitekit-components-gm2-********************.js	5.88 kB
./dist/assets/js/googlesitekit-components-gm3-********************.js	10 kB
./dist/assets/js/googlesitekit-data-********************.js	2.18 kB
./dist/assets/js/googlesitekit-datastore-forms-********************.js	9.1 kB
./dist/assets/js/googlesitekit-datastore-location-********************.js	2.09 kB
./dist/assets/js/googlesitekit-datastore-site-********************.js	19.1 kB
./dist/assets/js/googlesitekit-datastore-ui-********************.js	10.1 kB
./dist/assets/js/googlesitekit-datastore-user-********************.js	24.8 kB
./dist/assets/js/googlesitekit-entity-dashboard-********************.js	71.2 kB
./dist/assets/js/googlesitekit-i18n-********************.js	3.92 kB
./dist/assets/js/googlesitekit-main-dashboard-********************.js	95.7 kB
./dist/assets/js/googlesitekit-modules-********************.js	21.7 kB
./dist/assets/js/googlesitekit-modules-ads-********************.js	18.1 kB
./dist/assets/js/googlesitekit-modules-adsense-********************.js	110 kB
./dist/assets/js/googlesitekit-modules-analytics-4-********************.js	109 kB
./dist/assets/js/googlesitekit-modules-pagespeed-insights-********************.js	22.4 kB
./dist/assets/js/googlesitekit-modules-search-console-********************.js	58 kB
./dist/assets/js/googlesitekit-modules-tagmanager-********************.js	30.1 kB
./dist/assets/js/googlesitekit-polyfills-********************.js	379 B
./dist/assets/js/googlesitekit-settings-********************.js	59.7 kB
./dist/assets/js/googlesitekit-splash-********************.js	71.7 kB
./dist/assets/js/googlesitekit-user-input-********************.js	46.8 kB
./dist/assets/js/googlesitekit-vendor-********************.js	316 kB
./dist/assets/js/googlesitekit-widgets-********************.js	33.4 kB
./dist/assets/js/googlesitekit-wp-dashboard-********************.js	60.1 kB
./dist/assets/js/optin-monster-********************.js	633 B
./dist/assets/js/runtime-********************.js	1.29 kB
./dist/assets/js/woocommerce-********************.js	612 B

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