Backport PR 26985, upgrading indices with a custom similarity plugin

[thomas11]

This is a straightforward port of #26985 to the 5.6 branch. Quoting the original:

Fix upgrading indices which use a custom similarity plugin.
Use a fake similarity map that always returns a value in MetaDataIndexUpgradeService.checkMappingsCompatibility instead of an empty map.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[rjernst]

@elasticmachine test this please

[thomas11]

01:53:27 Execution failed for task ':plugins:ingest-attachment:thirdPartyAudit'.
01:53:27 > Invalid exclusions, nothing is wrong with these classes: [javax/xml/bind/Marshaller.class, javax/xml/bind/helpers/DefaultValidationEventHandler.class]

Looks unrelated to this PR?

[rjernst]

Yes, the failure is unrelated (it ran with java 9 instead of java 8). I will re-trigger once CI is fixed to run java 8 with all 5.6 PRs.

[thomas11]

Hey @rjernst, pinging you as you suggested earlier :) Any news on the ES 5 CI build?

[rjernst]

I'm told it should work now.

@elasticmachine test this please

[rjernst]

Thanks @thomas11