Revert parallelism threshold change #4830

Job	Run time
debug_all_tests (ubuntu-latest)	4m 55s
debug (ubuntu-latest)	4m 44s
release_all_tests (ubuntu-latest)	3m 51s
release (ubuntu-latest)	3m 40s
debug64_all_tests (ubuntu-latest)	4m 53s
debug64 (ubuntu-latest)	4m 15s
Release64 (ubuntu-latest)	3m 47s
Release64_all_tests (ubuntu-latest)	3m 46s
	33m 51s

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