Update needed: polkadot_api (1.17.2 -> 1.20.1)

A new release has been detected for polkadot_api.

**Category:** javascript_packages
**Current version:** 1.17.2
**Latest version:** 1.20.1

Please review the [changelog](https://www.npmjs.com/package/polkadot-api/v/1.20.1) and update the documentation accordingly.
