This commit removes the Index Audit Output type, following its deprecation
in 6.7 by 8765a31d4e6770. It also adds the migration notice (settings notice).
In general, the problem with the index audit output is that event indexing
can be slower than the rate with which audit events are generated,
especially during the daily rollovers or the rolling cluster upgrades.
In this situation audit events will be lost which is a terrible failure situation
for an audit system.
Besides of the settings under the `xpack.security.audit.index` namespace, the
`xpack.security.audit.outputs` setting has also been deprecated and will be
removed in 7. Although explicitly configuring the logfile output does not touch
any deprecation bits, this setting is made redundant in 7 so this PR deprecates
it as well.
Relates #29881
This documents how to include the search queries in the audit log.
There is a catch, that even if enabling `emit_request_body`, which should
output queries included in request bodies, search queries were not output
because, implicitly, no REST layer audit event type was included.
This folk knowledge is herein imprinted.
Albert Zaharovits