You maintain a workflow that tests a library across three operating systems (ubuntu-latest, windows-latest, macos-latest) and three Node versions (16, 18, 20) using a matrix. Your team pays for a limited number of macOS runners, so you must (1) skip the combination of macos-latest with Node 16 entirely, and (2) ensure that no more than 2 jobs from this matrix ever run concurrently. Which strategy configuration achieves both requirements?