Default jetty.*.acceptors should be 1

**Jetty version**
10.0.x

**Description**
Given that connections are more and more persistent (due to protocols keeping them open for performance reasons), and many many cores, I think the current heuristic is too generous (up to 23 cores yields 2, up to 31 cores yields 3, more yields 4), and a fixed heuristic of `1` is plenty enough.