feat(core, jdbc): use an elastic thread pool

[loicmathieu]

To avoid using too many threads when it's not needed.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
1638	1	1637	8

View the top 1 failed test(s) by shortest run time

io.kestra.runner.h2.H2ServiceLivenessCoordinatorTest taskResubmitSkipExecution()

Stack Traces | 2.05s run time

java.lang.AssertionError: 
Expected: is <true>
     but: was <false>
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
	at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:6)
	at io.kestra.jdbc.runner.JdbcServiceLivenessCoordinatorTest.taskResubmitSkipExecution(JdbcServiceLivenessCoordinatorTest.java:161)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at io.micronaut.test.extensions.junit5.MicronautJunit5Extension$2.proceed(MicronautJunit5Extension.java:142)
	at io.micronaut.test.extensions.AbstractMicronautExtension.interceptEach(AbstractMicronautExtension.java:162)
	at io.micronaut.test.extensions.AbstractMicronautExtension.interceptTest(AbstractMicronautExtension.java:119)
	at io.micronaut.test.extensions.junit5.MicronautJunit5Extension.interceptTestMethod(MicronautJunit5Extension.java:129)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

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