This commit introduces the formal notion of a private setting. This
enables us to register some settings that we had previously not
registered as fully-fledged settings to avoid them being exposed via
APIs such as the create index API. For example, we had hacks in the
codebase to allow index.version.created to be passed around inside of
settings objects, but was not registered as a setting so that if a user
tried to use the setting on any API then they would get an
exception. This prevented users from setting index.version.created on
index creation, or updating it via the index settings API. By
introducing private settings, we can continue to reject these attempts,
yet now we can represent these settings as actual settings. In this
change, we register index.version.created as an actual setting. We do
not cutover all settings that we had been treating as private in this
pull request, it is already quite large due to moving some tests around
to account for the fact that some tests need to be able to set the
index.version.created. This can be done in a follow-up change.

* master: (197 commits)
  Prevent NPE parsing the stop datafeed request. (elastic#33347)
  HLRC: Add ML get overall buckets API (elastic#33297)
  Core: Fix epoch millis java time formatter (elastic#33302)
  [Docs] Improve tuning for speed advice (elastic#33315)
  [Rollup] Fix Caps Comparator to handle calendar/fixed time (elastic#33336)
  [CI] Mute  IndexShardTests#testIndexCheckOnStartup fails elastic#33345
  [CI] Mute LuceneChangesSnapshotTests#testUpdateAndReadChangesConcurrently
  Security for _field_names field should not override field statistics (elastic#33261)
  Add early termination support to BucketCollector (elastic#33279)
  Fix extractjar task  ci  (elastic#33272)
  Mute testFollowIndexAndCloseNode
  Logging: Drop Settings from some logging ctors (elastic#33332)
  HLREST: add update by query API (elastic#32760)
  TEST: Increase timeout testFollowIndexAndCloseNode (elastic#33333)
  HLRC: ML Flush job (elastic#33187)
  HLRC: Adding ML Job stats (elastic#33183)
  LLREST: Drop deprecated methods (elastic#33223)
  Mute testSyncerOnClosingShard
  [DOCS] Moves machine learning APIs to docs folder (elastic#31118)
  Mute test watcher usage stats output
  ...