Increase iOS simulator child processes from 5 to 6. Our testing shows we have enough headroom for one more, so let's get the speedup.
<rdar://problem/107850108>
Pull request: https://github.com/WebKit/WebKit/pull/12585
Committed 262792@main (0c9e65e27d2b): <https://commits.webkit.org/262792@main> Reviewed commits have been landed. Closing PR #12585 and removing active labels.