Update prefixed CSS properties in about.css

[afercia]

On a clean, updated, trunk install:

• modify some CSS file so that postcss:core will run during grunt precommit
• run grunt precommit
• you will get a modified src/wp-admin/css/about.css file with some -moz prefixed properties removed

These changes should be committed to avoid unrelated changes in other commits.

[netweb]

At first glance, I thought this was a follow up to [42181] that was missed in the build process somehow.

Rather it's a change in behaviour via Autoprefixer/Browserslist configuration. Firefox v51 (the current is v58) requires the -moz-column-count: 2; prefix, column-count was ​added to Firefox v52 so I'm not sure as to why Autoprefixer/Browserslist is only fixing this now, I've spent 15 minutes looking and the rabbit hole is deep!

Let's get this in ASAP to "avoid unrelated changes in other commits" as @afercia has noted

[dd32]

Bumping, 4.9.4 has been released.

[SergeyBiryukov]

In 42687:

Build/Test Tools: Update prefixed CSS properties in about.css.

Firefox 52+ supports unprefixed column-count and column-gap properties.

Props netweb, afercia.
Fixes #43190.

[SergeyBiryukov]

Reopening for 4.9.5 consideration.

[SergeyBiryukov]

In 42802:

Build/Test Tools: Update prefixed CSS properties in about.css.

Firefox 52+ supports unprefixed column-count and column-gap properties.

Props netweb, afercia.
Merges [42687] to the 4.9 branch.
Fixes #43190.