Update to use PostCSS 8

[desrosj]

The CSS related build tooling currently uses PostCSS 6 or 7, depending on the dependency tree for each npm package. This ticket is to update these packages and the related tooling to use PostCSS 8.

In addition to the Core build tools, Twenty Nineteen also uses an outdated version of PostCSS and should be updated.

Trac ticket: https://core.trac.wordpress.org/ticket/57554

[TobiasBg]

#57567 was marked as a duplicate.

[desrosj]

In 55150:

Build/Test Tools: Update PostCSS to version 8.

This updates PostCSS related dependencies for the main Core build process to versions that use the latest major version of PostCSS, 8.x.

There were no changes to any versioned CSS files as a result of this update directly. However, two occurrences of -o-tab-size have been removed due to the version of caniuse-lite used being updated in the process.

See #57554, #57555.

[desrosj]

Sorry @peterwilsoncc, I missed the props in [55150] but I've gone and added it manually.

[55150] solves the update in Core build tools, but there's still some investigation needed into the resulting CSS changes in Twenty Nineteen. The PR has been updated to only include the Twenty Nineteen changes.

[costdev]

This ticket was discussed during the bug scrub. As this has commits during the 6.2 cycle and 6.2 Beta 1 is later today, I'll close this as fixed pending further work to address the issues in Twenty Nineteen.

Additional props to @mukesh27

[desrosj]

I'm going to reopen this because until the issues in Twenty Nineteen are addressed, it's not fully fixed.

This is a build tool related ticket, so fixing after the beta 1 deadline is allowed.

[desrosj]

In 55458:

Twenty Nineteen: Update PostCSS to version 8.x.

This updates the build processes within Twenty Nineteen to use the latest major version of PostCSS, 8.x.

There are no changes to any built files as a result of these updates.

Props desrosj, peterwilsoncc.
Fixes #57554.

Merged in https://core.trac.wordpress.org/changeset/55458.