Today we are lenient and we open an index if it has broken
settings. This can happen if a user installs a plugin that registers an
index setting, creates an index with that setting, stop their node,
removes the plugin, and then restarts the node. In this case, the index
will have a setting that we do not recognize yet we open the index
anyway. This leniency is dangerous so this commit removes it. Note that
we still are lenient on upgrades and we should really reconsider this in
a follow-up.

* master: (414 commits)
  Set ACK timeout on indices service test
  Implement byte array reusage in `NioTransport` (elastic#27696)
  [TEST] Remove leftover ES temp directories before Vagrant tests (elastic#27722)
  Cleanup split strings by comma method
  Remove unused import from AliasResolveRoutingIT
  Add read timeouts to http module (elastic#27713)
  Fix routing with leading or trailing whitespace
  remove await fix from FullClusterRestartIT.testRecovery
  Add missing 's' to tmpdir name (elastic#27721)
  [Issue-27716]: CONTRIBUTING.md IntelliJ configurations settings are confusing. (elastic#27717)
  [TEST] Now actually wait for merges
  Test out of order delivery of append only index and retry with an intermediate delete
  [TEST] remove code duplications in RequestTests
  [Tests] Add test for GeoShapeFieldType#setStrategyName (elastic#27703)
  Remove unused *Commit* classes (elastic#27714)
  Add test for writer operation buffer accounting (elastic#27707)
  [TEST] Wait for merging to complete before testing breaker
  Add Open Index API to the high level REST client (elastic#27574)
  Correcting some minor typos in comments
  Add unreleased v5.6.6 version
  ...