Consolidate `external` and `hideExternalIndicator` props into an `openInNewTab` prop or `target="_blank"`.

[tofumatt]

Feature Description

See: https://github.com/google/site-kit-wp/pull/7847#discussion_r1402782242. More info to be filled out…

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Acceptance criteria

Implementation Brief

Test Coverage

QA Brief

Changelog entry

[mxbclang]

@tofumatt Can you also please add a priority label for this one? Thanks!

[ivonac4]

@tofumatt Are you still planning to work on this? If not, could you please unassign yourself so someone else can pick it up? Thank you!