* [APM] Always get the root transaction when fetching trace items
Fixes#33210
* code tweaks
* displays message notifying user that trace items exceeds maximum displayed
* remove getTraceRoot query by adjusting the score and order of trace
items with no parent.id
* add `apm_oss.maxTraceItems` config options to control the number of displayed trace items
* changed config `apm_oss.maxTraceItems` to `xpack.apm.ui.maxTraceItems`
* added missing configs to apm settings doc and docker template
* minor code tweak
8b57570b26