perf: index in Item and Item Variant Attribute (backport #36133)

[mergify]

This is an automatic backport of pull request #36133 done by Mergify.
Cherry-pick of 04400eb has failed:

On branch mergify/bp/version-13-hotfix/pr-36133
Your branch is ahead of 'origin/version-13-hotfix' by 1 commit.
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You are currently cherry-picking commit 04400eb2e4.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/stock/doctype/item/item.json

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[frappe-pr-bot]

🎉 This PR is included in version 13.52.6 🎉

The release is available on GitHub release

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