Add a groups index to elasticsearch

What does this MR do and why?

Add a groups index to elasticsearch.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create the index (Rails console):

   rec = ::Elastic::DataMigrationService.migrations.find { |m| m.name == "CreateGroupsIndex" }
   rec.migrate
   rec.completed? # => true

2. Verify the index + mappings exist:

   curl -s "localhost:9200/gitlab-development-groups/_mapping?pretty"

MR acceptance checklist

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Related to #607015 (closed)