Fix thread leak caused by eager creation of `SentryExecutorService` in `SentryOptions` #6868

Job	Run time
system-test (sentry-samples-jul, false, true)	2m 26s
system-test (sentry-samples-spring, false, true)	5m 47s
system-test (sentry-samples-spring-boot, false, true)	6m 28s
system-test (sentry-samples-spring-boot-jakarta-opentelemetry, true, true)	9m 2s
system-test (sentry-samples-spring-7, false, true)	5m 32s
system-test (sentry-samples-logback, false, true)	2m 37s
system-test (sentry-samples-spring-jakarta, false, true)	5m 54s
system-test (sentry-samples-spring-boot-4-webflux, false, true)	6m 46s
system-test (sentry-samples-spring-boot-4, false, true)	7m 38s
system-test (sentry-samples-log4j2, false, true)	2m 40s
system-test (sentry-samples-spring-boot-jakarta-opentelemetry-noagent, false, true)	7m 44s
system-test (sentry-samples-spring-boot-jakarta, false, true)	7m 18s
system-test (sentry-samples-spring-boot-opentelemetry, true, false)	8m 9s
system-test (sentry-samples-spring-boot-jakarta-opentelemetry, true, false)	8m 37s
system-test (sentry-samples-spring-boot-webflux-jakarta, false, true)	6m 35s
system-test (sentry-samples-console, false, true)	2m 40s
system-test (sentry-samples-spring-boot-4-opentelemetry, true, true)	9m 10s
system-test (sentry-samples-spring-boot-webflux, false, true)	6m 28s
system-test (sentry-samples-spring-boot-opentelemetry, true, true)	8m 46s
system-test (sentry-samples-spring-boot-4-opentelemetry, true, false)	8m 59s
system-test (sentry-samples-spring-boot-opentelemetry-noagent, false, true)	8m 6s
	2h 17m 22s

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