fix: update item_tax_rate in backend (backport #44884)

[mergify]

Issue: item_tax_rate is only set from the front end.
If the Item tax template is updated from the backend incorrect taxes are calculated.

Steps on how to replicate this issue

• Create one item with only one item tax template
• Create a transaction for this item. This should auto-fetch item tax template.
• Unset item tax template from the item. Save

With unset, it doesn't unset item tax rate, causing incorrect final taxes.

Closes: #41081
Frappe Support Issue: https://support.frappe.io/app/hd-ticket/27836

backport-version-15-hotfix
backport-varsion-14-hotfix

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This is an automatic backport of pull request #44884 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of de54c0b has failed:

On branch mergify/bp/version-15-hotfix/pr-44884
Your branch is up to date with 'origin/version-15-hotfix'.

You are currently cherry-picking commit de54c0b41f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   erpnext/accounts/doctype/sales_invoice/test_sales_invoice.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   erpnext/controllers/taxes_and_totals.py

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