Mark RubySingleThreadExecutor as a SerialExecutorService #635

Job	Run time
Test isolated (ruby)	3m 43s
Test isolated (2.3)	4m 11s
Test without C extension	3m 19s
Tests: Ruby 2.7 - ubuntu	3m 33s
Tests: Ruby 3.3 - ubuntu	3m 23s
Tests: Ruby 3.1 - ubuntu	3m 17s
Tests: Ruby 3.0 - ubuntu	3m 19s
Tests: Ruby 3.2 - ubuntu	3m 29s
Tests: Ruby 2.6 - ubuntu	3m 39s
Tests: Ruby 2.4 - ubuntu	3m 28s
Tests: Ruby ruby - windows	4m 33s
Tests: Ruby 2.5 - ubuntu	3m 34s
Tests: Ruby truffleruby - ubuntu	5m 23s
Tests: Ruby 2.3 - ubuntu	3m 33s
Tests: Ruby jruby - ubuntu	5m 57s
Tests: Ruby 3.4 - ubuntu	3m 21s
	1h 1m 42s

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