Update FB GRAPH API version to v20.0

[rawdreeg]

Changes proposed in this Pull Request:

Update Facebook Marketing API to v20.0

The endpoints we use have not changed, so the application should behave as it used to without any additional changes.

• Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical.

Detailed test instructions:

1. Use https://github.com/woocommerce/facebook-for-woocommerce/wiki/Smoke-tests for testing instructions
   Make sure that logging is enabled in the plugin. After the tests, inspect the logs for errors.
   The use of v20 API will be clearly visible in the endpoint URL:

Additional information:

I created a related PR to update the api.woocommerce.com/integrations bridge. This will need to be reviewed to ensure that the connection flow also works:

https://github.com/Automattic/woocommerce-connect-server/pull/2487

Changelog entry

Tweak - Bump Marketing API version to v20.0

[ibndawood]

Thank you, @rawdreeg.