Build Executor is creating an excessive amount of threads

[Sanne]

This attempts to limit the build executor to a size dependent on the system, with a rough attempt at ergonomics.
More importantly, it allows setting system properties in the build to fully control the sizes, allowing further experimentation.

Fixes #49574

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit b28d21d.

✅ The latest workflow run for the pull request has completed successfully.

It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

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Flaky tests - Develocity

⚙️ JVM Tests - JDK 21

📦 extensions/infinispan-cache/deployment

❌ io.quarkus.cache.infinispan.InfinispanCacheTest.testGetAsyncWithParallelCalls - History

• expected: "thread1" but was: "thread2" - org.opentest4j.AssertionFailedError

org.opentest4j.AssertionFailedError: 

expected: "thread1"
 but was: "thread2"
	at io.quarkus.cache.infinispan.InfinispanCacheTest.testGetAsyncWithParallelCalls(InfinispanCacheTest.java:283)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at io.quarkus.test.QuarkusUnitTest.runExtensionMethod(QuarkusUnitTest.java:534)
	at io.quarkus.test.QuarkusUnitTest.interceptTestMethod(QuarkusUnitTest.java:448)