CSS line-height values should be unitless - dashboard.css

[ianbelanger]

As outlined in #44643, CSS line-height values should be unitless unless necessary to be defined as a specific pixel value. It was suggested that we break up 44643 by stylesheet in order to better track them.

This ticket covers much of wp-admin/css/dashboard.css, see patch notes below

Note: The patch was tested only on a Windows machine in Chrome, Firefox, IE11 and Edge. As per https://core.trac.wordpress.org/ticket/44643#comment:23 it should probably be tested on other OS's and supported browsers.

Patch Notes:

Unable to find selectors in the DOM

line 471 - #dashboard_php_nag .button .dashicons-external (not sure how to trigger php nag)

[ianbelanger]

Updates patch with more precise line-height calculations

[SergeyBiryukov]

In 45461:

CSS Coding Standards: Use unitless values for line-height in wp-admin/css/dashboard.css.

Props ianbelanger, pbiron, afercia.
Fixes #46510. See #44643.