Reduce parallelism of generated mainline fuzzer tasks

With the dedicated Query patch variants, we can make pre-commit testing more configurable for the fuzzers and rely less on post-commit fuzzer runs. This has the benefit of catching issues earlier and reducing the number of fuzzer BFGs.

This ticket will reduce the parallelization of mainline fuzzer runs from 5 to 1, consistent with other tasks running on mainline.

It also makes it possible to add more tasks before we hit the CI project config size limit