Reduce the number of parallel CIs to reduce flakiness

[RafaelGSS]

Hey folks,

It's not a new issue that CI tends to be flaky due to parallelization, for instance, tests that use more memory can cause OOM in a separate CI. Whenever a security release happens, we lock CI so only patches and proposals for the security release can trigger CI. The outcome is a fast CI and less likely to incur shared-resources errors.

Is there a place where we document the amount of test-node-pull-request that runs in parallel? Have we tried to reduce that number targeting more effective CIs?

Note that if reducing the number of parallel CIs results in less flaky tests, it will greatly optimize development. Instead of running 4 CIs until they turn green, we only need to run one.