Use `wp i18n update-po` for updating PO files

[strarsis]

This PR replaces the existing shell script with a wp i18n invocation for the newly added update-po subcommand that updates the PO files with the POT file.

This eliminates the need for external tooling (except for the wp CLI of course) and shell scripts.

The latest wp i18n version (>=2.4.0) is needed for the newly added update-po subcommand. Until wp CLI is also newly released with the new wp i18n version bundled, the latest wp i18n version can be installed independently from the wp CLI version by using wp package install wp-cli/i18n-command:dev-main and then just using it as usual (wp i18n).