Refactor tax rate storing

[bartech]

Description

Unifies how rates are stored. It only use itemized rates. Display tax totals still allow to control how rates are displayed in Cart and Checkout and both options save same tax rates. This allow to remove clearing of tax rates after Display tax totals is changed.

Removes get_combined_tax_rates and generate_combined_tax_rate_name methods.

Prevent saving "State" for VAT tax. VAT is country wide tax.

Related issue(s)

Closes https://linear.app/a8c/issue/WOOSHIP-1580/changing-display-tax-totals-clears-all-tax-rates-when-automated-taxes
Closes https://linear.app/a8c/issue/WOOSHIP-1569/no-vat-calculated-for-two-word-counties-in-lithuania
Closes https://linear.app/a8c/issue/WOOSHIP-1625/fatal-error-in-woocommerce-tax-taxjar-integration-conflict-with-square

Steps to reproduce & screenshots/GIFs

Checklist

• unit tests

• changelog.txt entry added
• readme.txt entry added

[dustinparker]

Everything appears to work as expected. I tested upgrading with several different scenarios and the rates are calculated and displayed properly. We just need to merge trunk into the branch and this should be good to go.

[iyut]

LGTM!