[APM] EUIfication of APM UI

[elasticmachine]

Concrete tasks

• Replace custom variables file with EUI vars:
  import * as euiVars from '@elastic/eui/dist/eui_theme_k6_light.json';
• Replace custom tabs with EUITabs
• Replace KUI Breadcrumbs with EUIBreadcrumbs – #25906

[elasticmachine]

Original comment by @formgeist:

@elastic/apm-ui Would love to have your input on the above, and please add more if there's anything I'm missing. Question is whether we can PR the changes, or you need me to create some visual examples of how we might combine certain components to create x.

[elasticmachine]

Original comment by @sqren:

Units
Move to Eui units system (resembles the existing for APM)
Colours
Move away from the existing variables.scss

We are currently waiting on javascript versions of the unit- and color-variables are available to us.

KUI -> EUI
I think we should start with the KUI to EUI migration. They are probably fairly easy.
We currently use KUI in these places: LINK REDACTED

[elasticmachine]

Original comment by @formgeist:

@sqren Super useful link that 👍 I'll make a note that we're awaiting js versions of unit and colour vars.

[elasticmachine]

Original comment by @sqren:

Use EuiLink for adding accessibility states like focus

Didn't you do this already @vanjacosic ?

[elasticmachine]

Original comment by @vanjacosic:

I went through and replaced all KuiLink's and <a>'s with <EuiLink>. Though I won't vouch for the accessibility 😇

[formgeist]

Closing in favor of #28928 and #25906