[EuiTabs] Deprecate 'xl' tab size

[ryankeairns]

It's both rarely used and, subjectively, larger than we would care to display in our UI.
This change would mean updating `EuiPageHeader' (i.e. downsizing the the two size options it provides).

The portion of the docs related to sizing state:

In general you should always use the default (medium) size. The small size is best for when placing inside popovers or other small containers. Reserve using the large size for when using as primary page navigation, like inside of EuiPageHeader.

That last point seems rather debatable that we want to continue using it this way. I don't recall many (any?) places where we do use tabs as the primary page navigation sans a page title. Seems strange, imo.

[ryankeairns]

1. Deprecate xl tabs.
2. Downsize the tab sizing used in EuiPageHeader from xl, l to l, m.

With the following regex search of the Kibana repo, I find 0 instances of direct use of the xl size

<EuiTabs[ \w=\{\}>\(\)\n;".-:]* size="xl"