Improve UI design for file breadcrumbs

What does this MR do?

Improves UI design for file breadcrumbs, introduced in %7.1 and then unchanged for years, by:

1. using the spacing (margin) specified in @gitlab/ui
2. using the font-color specified in @gitlab/ui

Screenshots (strongly suggested)

before	after
	(highlighting bar by selection is artifact)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.

Related to #330616 (closed) #330620 (closed) and #239865 (closed)