Update stylelint rule function-linear-gradient-no-nonstandard-direction to be consistent with Drupal's CSS standards

I'm pretty sure this rule can just be enabled now we have dropped support for old browsers.

This is enabled as part of stylelint-config-recommended, and we don't have any violations of it at present, so we can just remove the line that skips this rule.

@longwave solution seems straight forward and didn't cause any issue.

We can safely backport to 10.0.x as the standard applies there just fine - but it fails on 9.5.x so we can not backport there.

Committed and pushed d69b07715f to 10.1.x and 4ee7b62c28 to 10.0.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.