Initial commit here just for CI checking that forward-port of 6.x-based #27906 was successful. Once green I'll add a commit that changes the default for allowPartialSearchResults to false (meaning errors will be thrown by default when it looks like partial results would be returned)

Reverted the code that defaults searches to strict (error on partial results). Will open a separate issue once this is pushed to consider changing the default to strict.

CI build failures look to all be related to BWC checks with the 6.3 snapshot that doesn't yet have the new allowPartialSearchResults flag serializing in SearchRequest

If I can assist in integrating a change like this with cross-cutting BWC implications, please let me know. I am always happy to help.

@jasontedor Thanks for the offer of help. I have 2 PRs lined up ready to push (if I can get an OK on this one, @colings86 ?)
This PR is for master and is failing on BWC, but I have a seperate 6.x PR which is passing tests.
I thought the best way to sequence this might be to:

1. push the 6.x PR which should be green but will then cause all master BWC search tests to fail (master will not de/serialize search requests to 6.x)
2. push this master PR which should then hopefully be green

Not sure if there needs to be a gap between 1) and 2) waiting for the 6.x build to make it through the intake tests?

After all this is done I'll open a separate issue to open the debate on whether new cluster default setting is to fail on partial results.

@markharwood I think that this would help keep the build green every step of the way and let you integrate this at a leisurely pace:

• push a commit to this PR switching all version-dependent serialization logic to Version.V_7_0_0
• push a commit to this PR that disables BWC testing; make a change to the root build.gradle setting ext.bwc_tests_enabled to false in all projects.
• at this point, you should be able to get a completely green build
• merge this PR
• prepare a backport of this change to 6.x, changing the version-dependent serialization logic to Version.V_6_3_0, and setting ext.bwc_tests_enabled to true
• at this point, you should be able to get a completely green build on 6.x, and integrating this change there will have no impact on master
• backport this PR with the above changes to 6.x
• prepare a commit against master that changes the version-dependent serialization logic to Version.V_6_3_0 and reenables ext.bwc_tests_enabled; at this point you can run the BWC tests and they should be green
• integrate the above change of the version logic and reenabling BWC into master