Switch Service Account breadcrumbs to use singleNavOptIn

What does this MR do and why?

Switches the Admin Service Account pages to use the new opt-in way of using injectVueAppBreadcrumbs. See the linked issue below for more context.

References

Part of Migrate remaining Vue breadcrumbs to new inject... (#539583 - closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Check http://gdk.test:3000/admin/application_settings/service_accounts and find only a single GlBreadcrumb component containing all breadcrumb items.
• Compare to master where this page had two GlBreadcrumb components next to each other.

MR acceptance checklist

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