[EuiBottomBar] Redesign for improved contrast and fit with new chrome

[ryankeairns]

See this internal Slack thread

EuiBottomBar was intentionally designed to have pronounced contrast. Recent feedback surfaced that a) this design decision does not translate to dark mode (it blends into the UI; separate issue to be created) and b) that this design is visually too harsh.

Put another way, its worth revisiting and modernizing the visual design of this component especially as we update the Kibana UI and introduce a slot for a global bottom bar (there is debate on doing this; its in the layout but not exposed yet).

Some initial ideas on what to consider were shared in the thread. Here is one example:

TODO

1. Identify use cases as inputs to the design (namely, Stack Management > Advanced Settings)
2. Explore new designs
3. Share back, iterate (Obs team brought this up)

[patpascal]

Leaving here the test file https://www.figma.com/design/drEX6VmvbWrHZlWl7SsrFY/EUIBottomBar?m=auto&t=9s27hHBrzLP99YoS-6

GH Issue #9204