Less aggressive async pruning for RubyThreadPoolExecutor #528

Job	Run time
Tests: Ruby 2.3 - ubuntu	4m 57s
Test isolated (2.3)	5m 17s
Tests: Ruby 2.4 - ubuntu	5m 2s
Test isolated (ruby)	4m 59s
Tests: Ruby 2.5 - ubuntu	4m 51s
Tests: Ruby 2.6 - ubuntu	4m 53s
Tests: Ruby 2.7 - ubuntu	4m 50s
Tests: Ruby 3.0 - ubuntu	4m 44s
Tests: Ruby 3.1 - ubuntu	4m 36s
Tests: Ruby 3.2 - ubuntu	4m 42s
Tests: Ruby 3.3 - ubuntu	4m 39s
Tests: Ruby jruby - ubuntu	5m 4s
Tests: Ruby truffleruby - ubuntu	4m 16s
Tests: Ruby 3.3 - windows	6m 10s
	1h 9m 0s

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