Mute flaky delete_by_query conflicts=proceed case in multi-project core REST YAML suite

[Copilot]

CoreWithMultipleProjectsClientYamlTestSuiteIT intermittently fails on delete_by_query/10_basic/Response for version conflict with conflicts=proceed, with observed result inversion (deleted/version_conflicts/count) under multi-project execution. This PR scopes the fix to the multi-project QA harness by excluding only that unstable case from this suite.

• Problem scope

  • Flake is isolated to the multi-project core REST YAML run of a single delete_by_query conflict scenario.
  • Failure pattern indicates nondeterministic outcome for expected conflict accounting in this environment.

• Change made

  • Added one targeted blacklist entry in:
    • x-pack/qa/multi-project/core-rest-tests-with-multiple-projects/build.gradle
  • No production code or shared REST spec behavior was modified.

• Configuration diff (targeted exclusion)

  // These tests assume we are running on a single node
  '^delete_by_query/10_basic/Response for version conflict with conflicts=proceed',
  '^delete_by_query/50_wait_for_active_shards/can override wait_for_active_shards',
  '^update_by_query/50_consistency/can override wait_for_active_shards',

[elasticsearchmachine]

@copilot-swe-agent please enable the option "Allow edits and access to secrets by maintainers" on your PR. For more information, see the documentation.