Warn user when disabling elasticsearch indexing

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• Close this issue

• When disabling elasticsearch indexing, there could be unintended consequences, such as search results becoming erroneous and/or incomplete.
• More context in this issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/5299.
• Provide some mechanism to warn the user when they disable elasticsearch indexing, so that they know there might be some unintended consequences.