Now that warning headers no longer contain a timestamp of when the
warning was generated, we no longer need to extract the warning value
from the warning to determine whether or not the warning value is
duplicated. Instead, we can compare strings directly.

Further, when de-duplicating warning headers, are constantly rebuilding
sets. Instead of doing that, we can carry about the set with us and
rebuild it if we find a new warning value.

This commit applies both of these optimizations.

…ead-de-duplication

* elastic/master: (24 commits)
  [TEST] Mute MlMappingsUpgradeIT testMappingsUpgrade
  Streamline skip_unavailable handling (elastic#37672)
  Only bootstrap and elect node in current voting configuration (elastic#37712)
  Ensure either success or failure path for SearchOperationListener is called (elastic#37467)
  Target only specific index in update settings test
  Add a note how to benchmark Elasticsearch
  Don't use Groovy's `withDefault` (elastic#37726)
  Adapt SyncedFlushService (elastic#37691)
  Mute FilterAggregatorTests#testRandom
  Switch mapping/aggregations over to java time (elastic#36363)
  [ML] Update ML results mappings on process start (elastic#37706)
  Modify removal_of_types.asciidoc (elastic#37648)
  Fix edge case in PutMappingRequestTests (elastic#37665)
  Use new bulk API endpoint in the docs (elastic#37698)
  Expose sequence number and primary terms in search responses (elastic#37639)
  Remove LicenseServiceClusterNotRecoveredTests (elastic#37528)
  Migrate SpecificMasterNodesIT to Zen2 (elastic#37532)
  Fix MetaStateFormat tests
  Use plain text instead of latexmath
  Fix a typo in a warning message in TestFixturesPlugin (elastic#37631)
  ...

* master:
  Liberalize StreamOutput#writeStringList (elastic#37768)
  Add PersistentTasksClusterService::unassignPersistentTask method (elastic#37576)
  Tests: disable testRandomGeoCollectionQuery on tiny polygons (elastic#37579)
  Use ILM for Watcher history deletion (elastic#37443)
  Make sure PutMappingRequest accepts content types other than JSON. (elastic#37720)
  Retry ILM steps that fail due to SnapshotInProgressException (elastic#37624)
  Use disassociate in preference to deassociate (elastic#37704)
  Delete Redundant RoutingServiceTests (elastic#37750)
  Always return metadata version if metadata is requested (elastic#37674)

…ead-de-duplication

* elastic/master:
  Use explicit version for build-tools in example plugin integ tests (elastic#37792)
  Change `rational` to `saturation` in script_score (elastic#37766)
  Deprecate types in get field mapping API (elastic#37667)
  Add ability to listen to group of affix settings (elastic#37679)
  Ensure changes requests return the latest mapping version (elastic#37633)
  Make Minio Setup more Reliable (elastic#37747)