This local checkpoint tracker uses collections of bit sets to track
which sequence numbers are complete, eventually removing these bit sets
when the local checkpoint advances. However, these bit sets were eagerly
allocated so that if a sequence number far ahead of the checkpoint was
marked as completed, all bit sets between the "last" bit set and the bit
set needed to track the marked sequence number were allocated. If this
sequence number was too far ahead, the memory requirements could be
excessive. This commit opts for a different strategy for holding on to
these bit sets and enables them to be lazily allocated.

* master:
  Remove checkpoint tracker bit sets setting
  Fix stable BWC branch detection logic
  Fix logic detecting unreleased versions
  Enhances exists queries to reduce need for `_field_names` (elastic#26930)
  Added new terms_set query
  Set request body to required to reflect the code base (elastic#27188)
  Update Docker docs for 6.0.0-rc2 (elastic#27166)
  Add version 6.0.0
  Docs: restore now fails if it encounters incompatible settings (elastic#26933)
  Convert index blocks to cluster block exceptions (elastic#27050)
  [DOCS] Link remote info API in Cross Cluster Search docs page
  Fix Laplace scorer to multiply by alpha (and not add) (elastic#27125)
  [DOCS] Clarify migrate guide and search request validation
  Raise IllegalArgumentException if query validation failed (elastic#26811)
  prevent duplicate fields when mixing parent and root nested includes (elastic#27072)
  TopHitsAggregator must propagate calls to `setScorer`. (elastic#27138)