Regroup thread-related server properties under groups

[bclozel]

There are already several thread-related configuration properties for Undertow, Tomcat and Jetty. We should regroup those under dedicated threads group to better organize the configuration namespace.

We might also consider other related issues like #18489 at the same time.

[onobc]

@bclozel I have a PR almost ready to submit for this. Where can I find out more about Spring Boot property deprecation policy? Specifically, do I need to mark the older properties with @DeprecatedConfigurationProperty(replacement = "server.tomcat.threads.max") for example?

[snicoll]

@Bono007 no specific other than the usual deprecation: the existing property should still be bound and flagged as deprecated. Tests should adapt to the new scheme but you we need to keep a @Deprecated test that exercises the deprecated properties to validate they still work. Here is a concrete example.

[snicoll]

Closing in favour of PR #19475