mediawiki.ui: Use WikimediaUI color for `@colorTextLight`
Making use of slightly higher contrast `#54595d` WikimediaUI base
color for `@colorTextLight` instead of `#666` to address minor
accessibility concerns and consolidate colors further.
`@colorTextLight` is currently used mainly in Flow and in one instance –
`.mixin-mw-ui-anchor-styles-quiet()` in mediawiki.ui.
Both are only impacted positively from exemplatory visual diff testing.
Bug: T149768
Bug: T152025
Change-Id: I40a4b2600772c9ca874361f17480f6b46b140823