Moves all thread related server properties under a "threads" group for each server implementation that has thread related properties (tomcat|jetty|undertow).

The existing property accessors have been marked as deprecated. The old impl just delegates to the new one which allows us to test that fact in a deprecated test and then all subsequent downstream tests can rely on the new api (ie have deprecate test in a single place for each property).

@snicoll do the Boot docs get auto-generated from the @DeprecatedConfigurationPropertys?

Closes gh-18620

Comment From: snicoll

@bono007 now that your other PR has been merged, would you be willing to rebase your proposal and include the max-queue-capacity property that has been introduced for Jetty?

Comment From: onobc

@snicoll Absolutely. I will do that this evening. MR ready sometime in the next 24 hours.

If you need it before then let me know and I will make the time.

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@bono007 https://github.com/bono007 now that your other PRs has been merged, would you be willing to rebase your proposal and include the max-queue-capacity property that has been introduced for Jetty?

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Comment From: onobc

@snicoll there were a few comments in the original proposal that we never resolved. I am ok with the proposal as is - if you want anything changed just let me know.

Comment From: snicoll

Thanks again @bono007