[A11y] Review selected text background color for 3:1 contrast ratio to page background color

[1Copenut]

@cee-chen, @MichaelMarcialis, and I worked on a PR to improve ::selected text background contrast on EuiBadge. This change was made to help users tell at a glance which text was highlighted vs. what was not.

Michael brought up a great question about non-text contrast as part of the PR review:

Forgive a potentially silly follow-up question: Since color is our only means of indicating the text highlighting, does that mean that text highlighting background colors must have a contrast ratio of 3:1 with the page background (in addition to the 4.5:1 contrast between highlight text and background colors)? If so, do we need to update our general text highlighting styles as well?
#7752 (comment)

I responded that yes, there could be an adjacent color requirement that would line up with [SC 1.4.11: Non-text Contrast (Level AA). I would like us to consider a design review that:

1. Highlights each badge text by click + dragging or Ctrl + A
2. Grabs the badge's new background color for ::selected text
3. Measures that color against the page background color for color contrast ratio
4. Who Can Use is a great resource to copy/paste hex values and get contrast ratios out
5. Document the findings in a spreadsheet